Fellow Sunline Club members,
This is a friendly heads-up informational post to urge you to add a camper frame inspection to your annual maintenance plan. Once you know where to look, the inspection itself will only take 10 to 15 minutes the first time. Less time the next time; this inspection is only a visual task and takes no special tools other than a flashlight.
This information can also apply to someone looking to buy a used camper or looking at one you own of any brand. I am not an expert on trailer frames, nor do I create trailer frames for a living; I am only passing along some of my findings as a goodwill gesture to help you keep your camper going for a long time. I have seen enough cracked RV trailer frames to see patterns in how they cracked, and I will present the findings on some of the ones I have seen so you can check your camper or one you are considering buying.
Please do not take this message as a “the sky is falling” warning that all used campers have frame issues, as this is not the case. The number of camper frames with failures in the big picture is low compared to the total number of campers on the road today, but frame failures happen. Many camper owners may have never considered inspecting their camper frame as part of annual maintenance; this topic falls into an out-of-sight, out-of-mind thing. Since this is a Sunline forum we are on, I will talk mainly about Sunlines. The last Sunline was built in the fall of 2006; come this fall, the last Sunline ever built will be 18 years old. We have many club members camping with campers older than 18, myself included. If you have a camper of any brand towed over 5,000 miles, consider adding an annual frame inspection.
You often can’t control the bad roads you end up on while traveling with your camper. Many campers do not have shock absorbers that tame down the excessive frame flexing. Towing over small or large bumps of bad roads aggravates and accelerates the camper frame flexing, which, in time, leads to metal fatigue that will start and continue to crack a camper frame as the miles add up. When you inspect your frame, if you find small cracks during the inspection, that is the time to create a plan for when and how to address the problem. Correcting it before the next camp may be necessary if you find advanced damage; if you do not know what it is you are looking at, look for someone to help you sort out when to get it addressed. After your first inspection, consider the follow-up inspections as an end-of-season activity. This gives you time to create a get-well plan if you find something before the start of the next camping season.
What I will present can also happen to other brand campers in the same common high-flex areas. This is an industry-wide issue, not just Sunlines. I have several friends with different brands with the same problems as the camper towing miles add up.
When campers are towed, the mileage accumulates. The more miles towed; the more flexing occurs in the steel of the camper frame. Flexing of the camper frame is normal and on purpose in most cases. Metal fatigue starts when the flexing metal stresses exceed the fatigue limit of the metal and design used in the application. A metal fatigue failure is an accumulative situation where the metal flexes back and forth to the point a crack “starts” in the metal. The design, manufacturing process, or the metal's exposure during the flexing cycles all play into the starting of a crack. That crack can take a long time in some cases and less time in others to get started, depending on several factors influencing it. Once the crack has started, the crack can increase in size more quickly with each normal flexing cycle of the camper frame during towing. As an example, if you towed the same low to medium annual miles per year, using the camper in the same manner, it may take 5 to 10 years in some cases to start a tiny crack and only 1 to 3 years for the crack to increase significantly in size, which in time, can lead to a metal failure.
Several factors influence the advancement of metal fatigue. Any factor alone can create metal fatigue; some may take longer or never show up while you own the camper. The next person who buys the camper may see the issue. The more combined factors, at the same time, can accelerate metal fatigue. Here are “some” common factors that can start a crack in a camper frame. Not in any order of importance, see how your camper stacks up to these factors:
1. Consider the number of miles the camper was towed since it was new. The 5,000-mile mark I discussed is subjective, but failures have occurred during this mileage period when combined with other factors.
2. The road or off-road conditions the camper was towed over. Smooth roads flex the camper less than rough roads or off-road travel. Unlike campers in a seasonal campground, campers with very little mileage may never experience metal fatigue.
3. The amount of cargo weight added to the camper. All campers have a weight rating; unless you have weighed your camper loaded ready to camp, you do not know if you have a lightweight load, at the weight limit, or over the weight limit.
4. The size and adjustment of a weight distribution hitch if one was used.
5. The amount of rusting/corrosion of the frame in high flexing areas.
6. The design of the trailer frame and the suspension system. The camper owner has no control over this; however, in the RV industry, trailer frames are considered a light-duty application compared to a construction trailer hauling heavy equipment used daily. The RV application has several specific considerations regarding the design process; some are lightweight, ability to handle the weight and size ratings, lower cost, lower duty cycle, towability efficiency, and a limited design life of the camper are all parts of the process. Engineers can design a camper frame that will not fail in a long-life RV application; however, the cost may not be practical for the average RV’er in terms of both RV cost and the size and cost of the tow vehicle to handle it.
7. Damage to the trailer frame. Damage can come in many ways, and some are; corrosion/rusting of the metal, overweight loading, mega-pot hole-laden roads, the frame being stressed by dragging it on the road driving out of a high-up gas station, and any impact or situation to the frame which creates high stress in the metal it was not designed to handle.
As you can see, all the above can be an everyday situation when towing your camper. I did not bring up the expected design life of a camper frame, as Sunline is no longer with us to ask that question. And I have not yet heard a tangible answer from any other brand camper. I can say that a maintained and repaired Sunline can last a good long time. Our 2004 T310SR, which we bought in 2007 and was built in November 2003, is now 20 years old, has over 50,000 miles we put on it, and is still going strong. Yes, I have done some major repairs and upgrades that are documented here on our site. If I maintain and keep repairing it, I can see it lasting another 15 to 20 years and another 50,000 miles. We have new members joining all the time, some with 30 to 40-year-old Sunlines. Age alone is not a factor; the seven items listed above are.
If you want to read more on metal fatigue, I found this steel supplier site with a simpler explanation of metal fatigue. https://www.metalsupermarkets.com/what-is-metal-fatigue/
And if you want a deeper techno-involved reading on metal fatigue, see here on Wiki. https://en.wikipedia.org/wiki/Fatigue_(material) Engineering websites delve even deeper into this.
Now, what to look for when inspecting your camper frame. I will try to do most of this with pictures so you can see what a failure looks like. I will do this in separate replies to make it easier to read. We have the intro post, which you are reading now; the inspections Part 1 A-Frames, part 2, the suspension area, and Part 3 and Part 4 are how some of the repairs or preventive reinforcements were done.
Some of you reading this post may do your first frame inspection and find the start of something with frame cracking or advanced frame failing. If you discover something, feel free to ask about it, take pics, and post so we can see what you have. We are here to try and help. You are also welcome to ask questions or comment about anything presented. We can all learn from each other.
I will first show a few pics with labels on several camper frame parts or areas. As the post goes on, these same labels will be used in discussion, and the pics will help show what we are talking about. You can ask about your camper using these terms so we know what you are trying to describe.
The following picture is from our 2004 T310SR, a 10,000# GVWR-rated travel trailer with a super slide (coach and dinette slide). Note: Most 8,600# and 10,000# GVWR Sunline travel trailers with I-beam style main frame rail campers were built like these pictures.
This T310SR camper is one of the larger Sunline travel trailers. The same frame principles exist in all the Sunline frames; they may be made from different sizes, steel grades, and steel shapes, but the overall design concepts are similar.
This camper's A-frame area is made from a 6” tall x 8.2 #/ft. C channel attached to a 10” tall frame header. Sunline used several steel shapes over the years to make the A-frame; some shapes were C channels in various sizes, rectangular tubing, or other shapes. Sunline used what is known as a “through header” design A-frame where the A-frame rails protrude through the header in most cases, unlike other brands, that attach the A-frame, which is made separately, and then is attached as a unit under the main frame rails and header.
Here is a typical Sunline-style A-frame. The A-frame area is one of the high areas of flexure of a travel trailer camper where cracks in the frame may exist.
The next area of higher frame flexing is the axle and suspension area that attaches to the wheels and tires. This 2004 T310SR is a leaf spring tandem axle setup like many, but not all, Sunlines. The pictures show the original Sunline suspension attached to the main frame rails. This camper uses a 10” tall x 9 #/ft. I-shaped main frame rails are often called an I-beam in the non-tech world. Technically, it is a thin RV beam, MH beam, in the RV and manufacturing housing industry, or M beam (miscellaneous), and it is not a true ASTM, I, or S shape beam. We will call it an I-beam for camper conversations. The main frame rails are welded to the main frame header by the A-frame area. The main frame rails have several cross supports (not shown) running left to right between the frame rails along the length of the camper. The rear bumper attaches to the back of the two main frame rails.
View looking under the camper towards the left side suspension system.
View of some of the main frame cross-supports. This model has an enclosed tank compartment, which traditionally only was supplied on certain models of the larger campers.
Here are some views of T310SR looking down the I-beam frame at the suspension area.
The view is looking forward from the left rear wheel area.
View of left side looking towards the back of the camper at the rear axle area.
This is a 2004 T-2499, 7,000 lb. GVWR non-slide tandem axle travel trailer. For this model and manufacturing year, they used a 5” tall x 6.7 lb./ft C channel as the main frame rails and the A-frame on a leaf spring suspension. As you can see, the main frame rails are made from different steel shapes, but the hangers, axles, equalizers, and shackles are very similar to the larger T310SR camper.
Not all 7,000# GVWR class campers used this C channel approach; some years used rectangular tubing for the main frame and A-frame. In the model year 2005, there was a frame redesign to change to 6” I beam main frame rails with 4” channel iron A-frames. This frame redesign in 2005 also changed the 5,500# and 6,000# GVWR travel trailers.
Many 2004 and older 5,000# GVWR class travel trailers used a 4” C channel main frame rails with a 4” C channel A-Frame.
Sunline also made many single-axle travel trailers and fifth-wheel campers with different combinations of main frame rails and A-frame or pin boxes. While there is a difference in the axle, suspension, and hitch areas, the trailer hitch area, the A-frame or pin box area, and the suspension area are areas of high flexure when towing. These are the main areas the frame inspection will be looking at.
More in the next reply on inspecting the A-Frame area.
This is a friendly heads-up informational post to urge you to add a camper frame inspection to your annual maintenance plan. Once you know where to look, the inspection itself will only take 10 to 15 minutes the first time. Less time the next time; this inspection is only a visual task and takes no special tools other than a flashlight.
This information can also apply to someone looking to buy a used camper or looking at one you own of any brand. I am not an expert on trailer frames, nor do I create trailer frames for a living; I am only passing along some of my findings as a goodwill gesture to help you keep your camper going for a long time. I have seen enough cracked RV trailer frames to see patterns in how they cracked, and I will present the findings on some of the ones I have seen so you can check your camper or one you are considering buying.
Please do not take this message as a “the sky is falling” warning that all used campers have frame issues, as this is not the case. The number of camper frames with failures in the big picture is low compared to the total number of campers on the road today, but frame failures happen. Many camper owners may have never considered inspecting their camper frame as part of annual maintenance; this topic falls into an out-of-sight, out-of-mind thing. Since this is a Sunline forum we are on, I will talk mainly about Sunlines. The last Sunline was built in the fall of 2006; come this fall, the last Sunline ever built will be 18 years old. We have many club members camping with campers older than 18, myself included. If you have a camper of any brand towed over 5,000 miles, consider adding an annual frame inspection.
You often can’t control the bad roads you end up on while traveling with your camper. Many campers do not have shock absorbers that tame down the excessive frame flexing. Towing over small or large bumps of bad roads aggravates and accelerates the camper frame flexing, which, in time, leads to metal fatigue that will start and continue to crack a camper frame as the miles add up. When you inspect your frame, if you find small cracks during the inspection, that is the time to create a plan for when and how to address the problem. Correcting it before the next camp may be necessary if you find advanced damage; if you do not know what it is you are looking at, look for someone to help you sort out when to get it addressed. After your first inspection, consider the follow-up inspections as an end-of-season activity. This gives you time to create a get-well plan if you find something before the start of the next camping season.
What I will present can also happen to other brand campers in the same common high-flex areas. This is an industry-wide issue, not just Sunlines. I have several friends with different brands with the same problems as the camper towing miles add up.
When campers are towed, the mileage accumulates. The more miles towed; the more flexing occurs in the steel of the camper frame. Flexing of the camper frame is normal and on purpose in most cases. Metal fatigue starts when the flexing metal stresses exceed the fatigue limit of the metal and design used in the application. A metal fatigue failure is an accumulative situation where the metal flexes back and forth to the point a crack “starts” in the metal. The design, manufacturing process, or the metal's exposure during the flexing cycles all play into the starting of a crack. That crack can take a long time in some cases and less time in others to get started, depending on several factors influencing it. Once the crack has started, the crack can increase in size more quickly with each normal flexing cycle of the camper frame during towing. As an example, if you towed the same low to medium annual miles per year, using the camper in the same manner, it may take 5 to 10 years in some cases to start a tiny crack and only 1 to 3 years for the crack to increase significantly in size, which in time, can lead to a metal failure.
Several factors influence the advancement of metal fatigue. Any factor alone can create metal fatigue; some may take longer or never show up while you own the camper. The next person who buys the camper may see the issue. The more combined factors, at the same time, can accelerate metal fatigue. Here are “some” common factors that can start a crack in a camper frame. Not in any order of importance, see how your camper stacks up to these factors:
1. Consider the number of miles the camper was towed since it was new. The 5,000-mile mark I discussed is subjective, but failures have occurred during this mileage period when combined with other factors.
2. The road or off-road conditions the camper was towed over. Smooth roads flex the camper less than rough roads or off-road travel. Unlike campers in a seasonal campground, campers with very little mileage may never experience metal fatigue.
3. The amount of cargo weight added to the camper. All campers have a weight rating; unless you have weighed your camper loaded ready to camp, you do not know if you have a lightweight load, at the weight limit, or over the weight limit.
4. The size and adjustment of a weight distribution hitch if one was used.
5. The amount of rusting/corrosion of the frame in high flexing areas.
6. The design of the trailer frame and the suspension system. The camper owner has no control over this; however, in the RV industry, trailer frames are considered a light-duty application compared to a construction trailer hauling heavy equipment used daily. The RV application has several specific considerations regarding the design process; some are lightweight, ability to handle the weight and size ratings, lower cost, lower duty cycle, towability efficiency, and a limited design life of the camper are all parts of the process. Engineers can design a camper frame that will not fail in a long-life RV application; however, the cost may not be practical for the average RV’er in terms of both RV cost and the size and cost of the tow vehicle to handle it.
7. Damage to the trailer frame. Damage can come in many ways, and some are; corrosion/rusting of the metal, overweight loading, mega-pot hole-laden roads, the frame being stressed by dragging it on the road driving out of a high-up gas station, and any impact or situation to the frame which creates high stress in the metal it was not designed to handle.
As you can see, all the above can be an everyday situation when towing your camper. I did not bring up the expected design life of a camper frame, as Sunline is no longer with us to ask that question. And I have not yet heard a tangible answer from any other brand camper. I can say that a maintained and repaired Sunline can last a good long time. Our 2004 T310SR, which we bought in 2007 and was built in November 2003, is now 20 years old, has over 50,000 miles we put on it, and is still going strong. Yes, I have done some major repairs and upgrades that are documented here on our site. If I maintain and keep repairing it, I can see it lasting another 15 to 20 years and another 50,000 miles. We have new members joining all the time, some with 30 to 40-year-old Sunlines. Age alone is not a factor; the seven items listed above are.
If you want to read more on metal fatigue, I found this steel supplier site with a simpler explanation of metal fatigue. https://www.metalsupermarkets.com/what-is-metal-fatigue/
And if you want a deeper techno-involved reading on metal fatigue, see here on Wiki. https://en.wikipedia.org/wiki/Fatigue_(material) Engineering websites delve even deeper into this.
Now, what to look for when inspecting your camper frame. I will try to do most of this with pictures so you can see what a failure looks like. I will do this in separate replies to make it easier to read. We have the intro post, which you are reading now; the inspections Part 1 A-Frames, part 2, the suspension area, and Part 3 and Part 4 are how some of the repairs or preventive reinforcements were done.
Some of you reading this post may do your first frame inspection and find the start of something with frame cracking or advanced frame failing. If you discover something, feel free to ask about it, take pics, and post so we can see what you have. We are here to try and help. You are also welcome to ask questions or comment about anything presented. We can all learn from each other.
I will first show a few pics with labels on several camper frame parts or areas. As the post goes on, these same labels will be used in discussion, and the pics will help show what we are talking about. You can ask about your camper using these terms so we know what you are trying to describe.
The following picture is from our 2004 T310SR, a 10,000# GVWR-rated travel trailer with a super slide (coach and dinette slide). Note: Most 8,600# and 10,000# GVWR Sunline travel trailers with I-beam style main frame rail campers were built like these pictures.
This T310SR camper is one of the larger Sunline travel trailers. The same frame principles exist in all the Sunline frames; they may be made from different sizes, steel grades, and steel shapes, but the overall design concepts are similar.
This camper's A-frame area is made from a 6” tall x 8.2 #/ft. C channel attached to a 10” tall frame header. Sunline used several steel shapes over the years to make the A-frame; some shapes were C channels in various sizes, rectangular tubing, or other shapes. Sunline used what is known as a “through header” design A-frame where the A-frame rails protrude through the header in most cases, unlike other brands, that attach the A-frame, which is made separately, and then is attached as a unit under the main frame rails and header.
Here is a typical Sunline-style A-frame. The A-frame area is one of the high areas of flexure of a travel trailer camper where cracks in the frame may exist.

The next area of higher frame flexing is the axle and suspension area that attaches to the wheels and tires. This 2004 T310SR is a leaf spring tandem axle setup like many, but not all, Sunlines. The pictures show the original Sunline suspension attached to the main frame rails. This camper uses a 10” tall x 9 #/ft. I-shaped main frame rails are often called an I-beam in the non-tech world. Technically, it is a thin RV beam, MH beam, in the RV and manufacturing housing industry, or M beam (miscellaneous), and it is not a true ASTM, I, or S shape beam. We will call it an I-beam for camper conversations. The main frame rails are welded to the main frame header by the A-frame area. The main frame rails have several cross supports (not shown) running left to right between the frame rails along the length of the camper. The rear bumper attaches to the back of the two main frame rails.

View looking under the camper towards the left side suspension system.

View of some of the main frame cross-supports. This model has an enclosed tank compartment, which traditionally only was supplied on certain models of the larger campers.

Here are some views of T310SR looking down the I-beam frame at the suspension area.
The view is looking forward from the left rear wheel area.

View of left side looking towards the back of the camper at the rear axle area.

This is a 2004 T-2499, 7,000 lb. GVWR non-slide tandem axle travel trailer. For this model and manufacturing year, they used a 5” tall x 6.7 lb./ft C channel as the main frame rails and the A-frame on a leaf spring suspension. As you can see, the main frame rails are made from different steel shapes, but the hangers, axles, equalizers, and shackles are very similar to the larger T310SR camper.

Not all 7,000# GVWR class campers used this C channel approach; some years used rectangular tubing for the main frame and A-frame. In the model year 2005, there was a frame redesign to change to 6” I beam main frame rails with 4” channel iron A-frames. This frame redesign in 2005 also changed the 5,500# and 6,000# GVWR travel trailers.
Many 2004 and older 5,000# GVWR class travel trailers used a 4” C channel main frame rails with a 4” C channel A-Frame.
Sunline also made many single-axle travel trailers and fifth-wheel campers with different combinations of main frame rails and A-frame or pin boxes. While there is a difference in the axle, suspension, and hitch areas, the trailer hitch area, the A-frame or pin box area, and the suspension area are areas of high flexure when towing. These are the main areas the frame inspection will be looking at.
More in the next reply on inspecting the A-Frame area.