I would recommend starting by going around the entire interior perimeter of the trailer and verifying that the rest of the floor is solid. Areas of usual concern are behind the refrigerator, around the front door, under any windows. If you find too much rot, it may make sense to go for a shell-off floor replacement from the start.
Anyway, to answer your question, step 1 is to remove the lowest piece of trim that wraps around the rear of the trailer. This will expose the interface between the shell and the belly skin that wraps up to rivet to the shell, and between the banana wraps and the shell. You can then remove the bellypan/banana wraps--they are just held on with pop-rivets. The bumper trunk is all held together with pop rivets as well. If you find yourself drilling out any bucked rivets, you are going in the wrong direction. The bumper is just held on by two screws on the top and two on the bottom, that fasten the bumper to the ends of the frame rails.
Have a good look at the condition of the rear center panel, where it rivets to the rear steel hold-down plate. If you have serious rear end separation, the aluminum panel may be very corroded, and the rivets may be ripping out of the panel.