Same issue with mine this summer. Got to Fort Wilderness and the next morning discovered that the back 10 feet or so of my belly material had somehow worked loose and was hanging down on the street side, under my rear slide.
I'll tell you what I did, since I still had plenty of time to get it to the dealer... I duct taped that dang thing back up. So far, over 1500 towing miles later it's still holding fine. I'll have the dealer check it out once I'm done camping for the year. Not sure if that is a option for you. Likewise, mine seems to just have been laid in the "L" part of the frame, I can't really tell what (if anything) ever really held it in place to begin with.