When I previously removed the toilet and found what is in picture #2 I wondered what I could do, but this is what I found: Since the toilet was to be discarded I just drilled a long series of 1/4" holes, cut or sawed in between and got the whole thing out by using pliers, cutters, etc. to come up with a rough cutout above the floor as in the picture.
I found that the floor was fine, and fortunately the new flange with stub slid into the original rubber sleeve through the floor. Nice.
I had bought the taller of the Thetford toilets, with pedal flusher and a hand-held sprayer, but even so I wanted the toilet a bit higher, mostly because the pedal should clear the wooden grate on the floor.
The Thetford was placed on a piece of plywood to trace its base contour, 1/2" was added to the line to form a comfortable platform. It turned out that I need to double the thickness, so the total elevation became 1 1/2" from the floor. (Screwed them together after coating everything in epoxy resin and fiberglass mat). The outer top rim was contoured with a 1/4" roundover router bit. Screwed to the floor with SS screws, using thick poly construction cement to fill gaps and make for a stable intallation. The worthless, freely rotating original mounting bolts had to be cut off, ground down, and the holes filled with sealant.
That left a rough gap around the elevated platform which was filled with "Kitty Hair" which is an epoxy with lots of fiberglass strands already mixed in. This stabilized the rough edge of the fiberglass floor pan. I then used a Bondo-like filler (Formula 27, water proof) to fill the rest of the gap and contoured it with a teaspoon.
After mega effort in sanding this tough stuff smooth I primer it with Interlux Pre-Coat and finished it with a couple coats of Interlux Brightside. The result is shown in the picture.
Aided by the Valterra-inspired inspection cover behind the unit the whole thing can now be taken out in less than 5 minutes for further refinishing of the area.
As Gina will attest, the "Toilet from Hell" is a major sore point for the Burro
: too low, flush lever awkward to reach, valves mostly a one-season proposition, valve replacement in stock form close to impossible, etc. I'm hoping this mod will erase these memories.