The best way to check for rot is to use an ice pick along the interior edges of the wall. The Ice pick will tell if you have a soft perimiter on your floor. You will have to get into cabinets and under beds to really check, but it is the place to start. The only other way to get access to the floor edge without tons of disassembly is by removing the lower rub rail moulding.
When you checked the level of the frame at the tounge, wheels and bumper, were the stabilizers down? Does putting them down have any effect on the interior floor? What is your floor made of? In the Argosy they tried all kinds of stuff. There are trailers with alumimum floors, and they act a bit diffrent than a traditional floor.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato