As I guessed, it sounds like a lot of weight was put on the rear shell. A couple extra-large guys, a snowload, or whatever. At this point, it doesn't really matter.
For an attempt at repair, I think I would
1. Open the front shell fully
2. Open the rear shell a couple inches
3. Climb inside over the hitch and propane area
4. Put a sturdy prop under the center of the rear shell, right at its front edge
5. Put some serious weight on the left and right edges of the rear shell, right at its front edge.
Perhaps it will bend back a bit toward the normal shape. Assuming it is an aluminum frame, this should be safe as long as you don't try to bend it too far. If it appears to be helping, proceed incrementally, but watch carefully to see if the aluminum skin is detaching from the frame.
As always, check the Trailmanor Technical Library for questions like this. In this case, start here
http://www.trailmanorowners.com/foru...ad.php?t=12588
Bill