[JRTJH]

slow,

I thought about that possibility, but "skirted around it" because I don't know how the OP maintains his trailer. I'd hesitate to suggest it's from a leak along the molding at the roof/rear wall union or around a window and make him feel that I'm suggesting he doesn't perform careful inspections and reseal any questionable areas. Some people are "on the roof inspecting" after every trip and some people don't know how to even get on their roof. Most of us are "somewhere in the middle".....

It's entirely possible that there was a pinhole leak in the roof that allowed water to leak into the inner surfaces and when the sun baked the rear wall, it was actually worse than having a cover/humidity problem... Then again, a leak at a tail light could allow moisture to collect, and since "heat rises" most of the damage would probably be seen at the top of the rear wall... Or a window leak, or ?????

But without knowing his maintenance practices, I wouldn't feel comfortable suggesting that not inspecting for leaks might have caused the problem, that wouldn't be fair to him without knowing more information that we have in this thread.....