On the frame issues, there are 3 of them, but more so 2 main ones that show up more often.
1. Starting with the 2005 models there was a main frame rail, header and A frame change in the 7,000# and some of the 5,500# GVWR trailer frames. It may be all of the 5,500# frames I’m not sure. This frame issue surrounds the A frame header bending issues. The problems surfaced on 2005 and up to some of the 2007 T2499's as the worst ones but other models were not exempt from it. This was due to the high tongue weight using a WD hitch that could be reached when the camper was loaded, using higher rated WD bars then needed on a smaller camper or a large WD hitch on a heavy truck not adjusted correctly.
The 2004 and some years older 7,000# frames had 5" channel iron main frame rails and a 5" channel iron A frame. I do not know how far back the 5" channel iron went but the header problem never showed up that I know on them. The frame header was also a little stronger due to thickness and flange dimensions depending which way the bending forces where applied.
The 2005 redesigned frame went to a taller 6" tall thin I beam main frame rail I "believe" to help main frame sag. They then made the A frame smaller with smaller 4" high channel iron A frame. Not sure why they did the 4” down size change, maybe to save weight since the taller main frame weighed more. The frame header was then 6" tall but the flange legs smaller and the metal may have been slightly thinner. By the end of the 2007 models they figured it out how to stop the problem of the header buckling. Club members who reinforced the frame before mega damage happened, have been successful in not having further issues or at least they have not reported them.
This header bending issue comes from using a WD hitch which most all campers need. The chain snap up brackets or L brackets pending hitch brand, pull down and out and try to twist the A frame side rails. Since the 4" channel is not as strong in twisting mode as the 2004 and older 5" channel iron A frames, the 4" A frame rail can twist too much if the forces are large enough in a turn when the WD bars are pulling on it. What happens when the twisting force is too large, the main A frame header can buckle in the middle from the A frame rails trying hard to resist all that force. In real bad cases the battery rails twist open.
Heavy tongue weight campers with WD hitches, WD hitches with very high WD bar ratings above the loaded TW and heavy truck bed loads when the WD hitch is not setup right can present higher twisting forces in the A frame at the chains area when the making a turn. Going straight ahead is not the problem. And in some cases, even 750# WD bars in a large compound angle turn can create high chain forces in a turn. The higher forces only become a problem when the A frame side rails cannot handle that higher force.
Here is a pic from the old T2499 thread. This is my prior T2499 with the 5" non-problem A frame rails. The curved arrows show the twisting forces I'm talking about.
Here is a top view of a snap up bracket where the chain forces want to exert large twisting forces at the top of the A frame rail.
And here is Larry's frame before he had it all fixed up. A string pulled along the bottom shows the outside part of the header dropping down as the center area buckled inward.
2. The hanger issue, is a problem on many of the 8,600# and 10,000# TT's which use a 10" tall thin I beam shape frame rail. This is a known problem across the RV industry now and is not unique to Sunline. Lippert even has a spec sheet on how to fix the problem.
The real issue is the lower flange of the I beam is very weak in twisting. The weak lower flange bends easily when the camper turns when it is not reinforced to prevent hanger twist. Sunline used longer spring hangers in many cases which can aggravate the problem. I had the problem in the infancy stages but I fixed it a long time ago before it cracked the main frame.
When the camper turns, the tandem wheels drag around a turn. This natural dragging of that axle setup creates large forces in the front, equalizer and rear spring hangers trying to resist the bending. Between the longer hanger and the weaker bottom flange of the main I beam frame rail when it is not reinforced, the web section above the hanger flexes hard enough it cracks the web.
Here is a random I beam web crack above the hanger I found on the web. This is from an Open Range camper. Heartland has the issue as well as others with unreinforced I beam frames.
Open Range RV Owners Forum
The horizontal rust line above hanger is a dead giveaway of the problem. The rust line is from the flexing of the web enough until it cracks. Then unpainted metal rusts and lets you know you have the problem. I have seen a few Sunlines with this issue and other brands.
Here is the LCI Lip sheet on the correction. You need the web plate to fix the cracked web and the hanger cross members to stop the hanger flex. I also see adding a in place hanger stiffener that Sunline did add on the 2006 T264SR’s.
Here is a post on how I prevented the problem once I realized it. Have had no issues since. I sort of did it in phases starting in 2009.
3. The last frame issue comes from mega pothole damage on a heavy camper. The link Sunline Fan listed above shows the problem in detail and correction. I hit a real bad patch of Interstate in NY on I-88 before it was fixed and could not exit the highway soon enough before the damage was started.
The long overhang of the heavy camper and the extreme bouncing buckled the lower flange of the main frame rail behind the rear axle hanger and in front of the front axle hanger. Once the lower flange is compromised even a small amount, normal towing keeps bending it a little more each time going over normal bumps in the road. Mine is the only Sunline I know of that had this issue. There are other brands with the problem but not as many as no. 2 above.
Hope this helps