Do a search for (dreaded) "rear end separation." Rotting rear-end floors and rear end separation go hand-in-hand, especially on 70's era trailers.
So best case scenario, all you have wrong is some rotted plywood, and you can scab in a patch.
Worst case scenario, your rear-most sheet of plywood will be so rotted that you will conclude that you might as well replace the whole sheet. This can be done without lifting the shell, but it will be a PITA. When you remove this sheet, you will find that your rear hold-down plate has rusted into near non-existence, your rear most frame cross-member will be all but missing, and the last 2-3 ft of frame will be cancerous with rust. The relative motion of the shell and frame will have caused the rivets in the bottom of the rear panel to begin tearing out as well.
My '73 had some floor rot in the rear that I didn't even notice when I bought it, and I ended up doing a shell-off.