Sounds like your "front plate" may be loose or rusted through. The plate is a steel reinforcing piece behind the skin, it is bent 90 ° at the bottom and fits between the floor and frame, the floor channel is thru-bolted to the frame with this in between. The plate runs up the front about 18" and is buck riveted to the outer skin during constuction, the outer skin in the front is the only .050 thick aluminum on the coach. It's easy to see any frame/body sepeartion when you move a coach with a forklift, you can bounce it up and down and really notice it. In your case you should be able to lock the coupler down, raise the jack and have someone stand on the rear bumper and lightly move up and down, if you see the body lifting away frome the frame you've found the problem. Then the fun begins, but it is repairable.