The front passenger side of RV body has started to sag. I thought maybe the screws that go through the exterior body wall to the floors have bent. So I jacked up the body shell and replaced the screws...
Shasta/Coachmen of that vintage could be a sandwich of sheet metal, foam plastic, and oriented-strand board. Stiffness would be provided by the OSB, so if that has been water damaged, actual sagging of the floor would be one of your possibilities.
But also check out the issue of spacers between the chassis and floor frame, because if one of the spacers has broken down or crumbled, the whole house can sag in that direction.