You could secure the shell to the frame by using some strips of plywood that were the same thickness as the original floor. You would need to remove the bolts (which are probably rusty anyway if the floor rotted out) slide the strips of plywood into the c-channel and install new bolts through the c-channel and strips of plywood and into the outriggers. That should get you home.
1946 Spartan Manor
1965 Avion C-10 Truck Camper
2005 F-350 SRW 4x4 crew cab long box