Claire, I have put much thought into this question, long before you asked it. What I am about to say is strictly speculation.
It would make sense for the plywood in the floor to be bonded to both the fibreglass that covers the top and the bottom of it. You would be further ahead if the plywood had disintegrated. That way you would not have to worry about getting it out.
The quick and dirty solution is to cut a sheet of Ĺ" plywood to cover your floor. That will spread out the stress so that the soft spot will not be as noticeable. Put a carpet over the plywood and enjoy your camper.
As the soft spot grows, and becomes a problem, in my opinion, the best approach would be to remove the coach from the frame, and cut off the bottom, from the outside. This makes it un-necessary to butcher the furniture inside the trailer. I would cut along the bottom of the, “pontoons” from where your front dinette exposes the plywood to the wheel well and around where the inside of the wheel well to where it meets the rise in the floor to the rear dinette floor level. That should be where that piece of plywood ends. If the bottom of the trailer doesn’t just fall
off at this point, then some of the plywood is still intact and still glued to both the top and bottom. This is where I run out of ideas. I was thinking a long rope saw, but my short search for one came up with a maximum length of 48”, (https://www.amazon.com/dp/B0878282DR
). This may be long enough, maybe not. A long wood blade on a reciprocating saw might help, but if the intact wood is in the middle of the cut section, then it may be necessary to cut smaller pieces. In the end this butchery would be on the bottom of the trailer, and not seen by most people.
Once the rotten plywood is removed, I would replace it with 1” thick, pultruded grating, (https://www.grainger.ca/en/product/I...FT-/p/WWG4AUC3
). It is expensive, and ľ” too thick, but it would never rot again.