Hello from Colorado: I have a 75 Overlander 27', two sizes shorter than your top of the line
Sovereign. I have the belly pan down now. I'm repairing the rear floor rot and rear end separation as well as installing new waste water holding tanks. Maybe my project thread will interest you.
I highly recommend you drop the belly pan to replace the subfloor. Gutting the interior is the hard work, the belly pan is easy. The belly pan is cheap aluminum, likely corroded and needing replaced anyways. But most importantly it will give you access to the outrigger bolts, elevator bolts, drain water plumbing, some wiring, and a bunch of other stuff. You can also thoroughly inspect the frame for cracks and buckling, a problem in the longer 70s trailers like ours. You might even get all ambitious and clean and paint the frame to help prevent future rust.
Just plan on placing your trailer on very sturdy jack stands about 20" (45 cm) off the ground. I use a mechanics roller to scoot around under the trailer. I have worked on gravel before by laying on a piece of old carpet. Drill out the old rivets and let the belly pan drop. Get ready for some really dirty insulation to fall in your face. You will wash up just fine. It will cost about $400 for replacement belly pan aluminum.
Once the belly pan aluminum is down, you can grab the elevator bolts (actually screws) from below and "wiggle" them until they break. They you can just push them up and out. The hole saw method works too, it is just more work than "wiggling" them back and forth.
You can replace the subfloor without lifting the shell. You will need to mostly release the body (shell) from the frame, but not from the "F shaped aluminum channel" between the exterior and interior skins. You will have to gently lift the body just a bit in order to slide a new piece of plywood subfloor in place. I think some folks slide in the new plywood from the outside.
You will likely get more advice from folks who have actually done what you want to do.
David