I'll share what I have learned since the T2499 thread years ago. The issue of the bent and broken frame header can and has happened on other floor plans other then the T2499 that use the 4" channel iron A frame. The 2005 to 2007's T2499's before Sunline figured out the final fix, had the issue show up early in their life. Other Sunline floor plans, took many more years for the problem to show up. Same issue, it just took longer to occur. I'm not seeing this as just a 2005 to 2007 issue, the science at the moment says it can happen on older Sunline too if the conditions are right.
But, as I said, other floor plans then the T2499 have had the issue. I myself have seen 3 of those failures in the last 2 years and I repaired one of the 3 in my shop as I now own it. Keep this in perspective, this does not mean all Sunlines that have a 4" A frame will have this issue, it takes a set of conditions to allow it to happen and all the conditions have to align at the same time and repeat a number of times before failure occurs. This is what I have found.
1. The loaded tongue weight can be high. High is considered between 750 to 1,000# or higher. This has to align with no. 2 and 3 below to be a problem.
2. The owner is using a WD hitch. Odds are higher for a failure with larger WD bars, 1,000# and up bars but we have had one member reported with 750# bars have the issue. There are subsets to the WD factor.
2A. The WD being set too aggressive to drop the front end of the truck will aggravate the issue.
2B. WD is used to shift truck bed weight that is not part of the camper tongue weight can aggravate the condition. (WD bars loaded more than normal TW) Example, the owner added extra firewood on several camping trips aft of the rear axle after the original WD settings were made.
3. Miles traveled and areas traveled when item 1 and 2 are at their higher end. If the camper is towed on level flat ground, the risk is lowered. A lot of uneven ground aggravates the issue especially when turning.
(from my opinion and what I have seen. )
The 4" channel iron used is strong enough in the vertical direction for campers in the 5,000 to 7,000# GVW area. However, the channel iron shape has limitations in how much twist it can handle. If the twisting load is low, there is no issue. If the twist loads are high, that excess twisting has to be resisted by the frame header.
The frame header design has not changed a lot over the years on many brands. And Sunline used the same header concept for many years without large scale issues. The Sunline header was cut at the bottom to allow the channel iron to pass through it during fabrication. The header was also shorter in height in some cases. That cut is not a problem by itself, but when excessive twist from the A frame rails comes with that design, the header can buckle in the center and/or bend the header down pivoting at the outside edge of where the A frame goes through the header.
The 2004 T2499's had 5" channel iron A frame rails and it has a larger strength in resisting twisting for the imposed loads then the 4" channel using the same header design. My big T310SR having a GVW of just under 10,000# has a 6" channel iron A frame with the same cut bottom header. But, 6" channel being thicker and heavier has a much higher resistance to twisting then the 5" even and has no issues with the heavy 1,600# loaded TW I have.
The end result issue:
When using a conventional type of WD hitch (excludes the PullRite hitch) the WD bar chains or L brackets hang on the top side of the A frame. In a turn on uneven ground, one side of the A frame is loaded heavily by the TW of the camper and that sides WD bar while the opposite side of the A frame is unloaded at its WD bar. The entire A frame is trying to be twisted. The WD bar on the loaded side pulls on the chain bracket or L bracket at the top and tries to twist the A frame rail outward at the top.
If the A frame twists enough, the header has to resist this high twisting load that is putting large forces presing inwards towards the center of the camper. If it cannot handle the twist, the header starts to buckle in the center and or the part of the header past the A frame rails starts to bend down.
Here is a picture from my prior 2004 T2499 with arrows showing the force directions I'm talking about. There was no issues with this camper and it had a 1,200# loaded TW.
Here is one of our members T2363 with a buckled center
Here is my 2004 T2475 with a bent header. A string pulled along the bottom to show the bent header
The amount of bend in the center
A slight buckle in the center
A buckle/starting to separate the header connection at the A frame rail.
How I repaired the issue. Added angle iron reinforcement to the A frame at the snap up brackets to help stop the A frame twisting.
After jacking the header back close to straight, added rectangular tubing reinforcement along the bottom of the header to tie the main frame rails, the A frame rails and the header all together to resist the twisting forces.
Again, this all comes down to circumstance. My 2004 T1950 had 1,000# WD bars on it from the prior owner. It made it across the county at least 2 times. It also sat parked for 8 years too until I bought it. The A frame and main frame rails are 4" channel. The header is straight as an arrow. I will reinforce it before I start towing it as a precaution.
A year ago, I inspected a 2005 T2363 for a friend up in MI. The header had failed/cracked at the A frame connection. It was close to seperation. Odds are high the owner never even knew about it. The dealer didn't either until I pointed it out. Unless you look for it, or that your battery drops some as that gets affected too, you do not always know the problem is occuring.
I also have a 2005 T2363 I restoring now for a friend in NY and his header is straight as an arrow. He bought it new and has 1,000# WD bars. He camps a lot, just short trips.
All this said, our forum member EMAN, had a 2007 T2499, it was one of the fixed 4" A frame/header setup's by Sunline. He also welded on a heavy angle iron battery bank for 4 AGM batteries and 2 bicycles on the A frame and toured the west off road in some cases for a few years with no issues. He was sort of our test case not knowing it at the time, where if you can stop the A frame twisting, the 4" channel can work under heavier than normal loads.
What exactly were you planning on adding to the A frame?
Hope this helps