Step 1, evaluate your entire floor, and decide how much patching/replacing there is to do.
Step 2, open up the rear bumper trunk and remove enough aluminum to get a look at the rear of the frame.
Step 3, If frame is excessively deteriorated and enough floor is rotted that you will need to replace entire sheets, remove the center rear most interor skin and evaluate the condition of hte insulation and hold-down plate (it may have rusted completely away).
Step 4, Take a long sip of your favorite adult beverage and make the tough decision. Do you have the unholy trinity (rotting floors+disintegrating frame+rodent infestation in the walls)? If so, go ahead and build the gantry frames and resign yourself to doing a shell-off. If things can be corrected with patch-work, then proceed with the patching.
Some people try to do complete floor replacements without lifting the shell. I promise you that this is the hard way. If you lift the shell, then you can do all of the frame, bellypan, and tank work with the frame flipped in whatever orientation is easiest for you to work. Otherwise, you will be spending a lot of time on your back under the trailer.
Once your floor and frame are done, you can start worrying about the interior color scheme.