Justin, I merged your duplicate posts into one. Fixing floors is a tuff job. If you look at some of the Sunrader Brochures you will see a description on how they are laminated together. The top layer is 3/8 inch plywood, then 1/2 inch foam then a skin of aluminum on the bottom. After time the layers delaminate which allows for even more flexing. Mine was also like yours but not as bad. What I did was remove all the carpet then lay down 1/2 inch plywood on top of the existing floor. One thing that I did not do was ad some bracing. If I have to do it over I will add some 2x4's on the under side and screw down from the top into the 2x4 creating a sandwich so to speak. As far as reattaching the fiberglass to the plywood I would have no idea. A boat repair shop might have a good idea for that. Anyhow putting the 1/2 inch ply down made a big difference for our rig. I did not glue it down in case one day I need to take it out.
Greg