If you can remove the FG furniture modules, get rid of every bit of subfloor above the fiberglass belly. I found mine to be affixed with resin, but given how you've described the condition of yours, I'm guessing you'll be able to pull it off relatively easily. A "multi-tool" like the Fein MultiMaster will make the job go even quicker. In any case, get rid of all the old material, and sand away residual resin or debris as necessary.
Ironically, because the PP is built with two fully molded "hemispheres"—the bottom shell creates a bathtub, trapping any water inside. (It's possible that the added weight
of water and the soggy subfloor in yours distorted the fiberglass belly over the years.) So as you make your repairs, you must ensure that there are no leaks
from above or penetrations from below. There should be 8-10 fasteners holding your shell onto your frame. These are no doubt rusted beyond recognition but you'll want to cut any remnants of these away. (Obviously, once free, you have the option of jacking the shell up and off the frame. The frame is most likely to be pretty rusty too.)
I would use a decent 5/8" plywood throughout to replace your original floor. (Definitely, jack up the sagging underbelly to make it as flat as possible.) When I do a restoration, after any necessary repairs and a thorough prep, I coat the fiberglass interior with fresh resin or a two-part epoxy paint
. I do the same thing to the replacement floor, coating all sides and the edges. (This is, admittedly, a bit of overkill since presumably there's no chance of water saturating the floor again. Right?)
If you manage to get the FG belly flat enough, you can use a good adhesive (I've had luck with Liquid Nails, for example) and glue your plywood down to the FG. Your 8-10 replacement hold-down fasteners should be enough to squeeze everything together. (You can coat your newly drilled holes with resin or epoxy before winding.) If you can't eliminate the sag, you may have to run screws from below, up through the FG and into your floor, to make things snug. Of course, you'll want to seal these holes and use stainless fasteners. If you are really ambitious, you could now "glass in" the floor along the edges to the shell walls.
Once your new subfloor is in and secure, it should be ready to re-install the furniture modules, as well as your new top flooring material.