Your original assumption is correct, as long as the roof seams and other critical joints are maintained, you shouldn't have any issues.
If my 1971 Starcraft would have been sealed with modern sealants and maintained, there would have been zero damage to restore. Since that wasn't possible there were certain areas that were damaged by continual leaks. The pictures you see on the internet of rotting wood are trailers that have not been maintained and have been left to suffer continual water damage. Even my Starcraft was not damaged 100%, most of it was as built.
Foam insulation is a boon to RV'ing as it not only has higher R value but adds to structural strength.
