So sorry you found water damage in the subfloor. We were very fortunate our '79 Sovereign
was bone dry. There have been many vintage owners who have created threads concerning subfloor replacement. But if you're having difficulty sorting out all the info, or get lots of conflicting advice, don't be afraid to consult with professionals. We had our new axles installed at Bob Jones RV here in South Houston and ordered the new Dometic 434 stove from them, along with a few other odds and ends. They are a repair and restoration shop who will work on any RV, but specialize in AS. Sometimes when we run into something we can't solve, we consult with them. They're happy to answer questions, because we are clients and have referred others to them. If you start with them they might be able to refer you to a professional in your area.
A couple of things to consider, and you will have this with many projects. Take a step back and compare the amount of time doing a patch job takes vs. replacing entirely. It might be far easier to just replace the entire subfloor than to shoehorn in new plywood. If you're going to be putting in new everything else, why not start with a clean slate from the bottom up? As far as the belly pans are concerned, if you haven't done so, get underneath and take a good look at them. You may find there are some patches which can be removed to give you access. That was the case with our trailer.
It may or may not be a dirty secret, but as much as you think you'll stick to a budget, it ain't gonna happen. Restoring one of these old girls is a labor of love. Nothing practical about in the least. But that's why some of us love them.