Lindy: There is no right or wrong way to do your floor. (Nobody's supposed to live in a bus in the first place!) There is almost certainly a ton of moisture in the plywood. I recently cut a couple of "donuts" out of Millicent's floor, and they were soggy. I would be more concerned about the long term health effects of mold and mildew than about falling thru the floor one day.
Considering the brutal difficulty of removing the plywood -- and the high risk of getting infected by rotting splinters while doing it, I would probably be inclined to suggest that you lay down plastic and new flooring on top of the old. ...Then sleep like a baby without a worry in the world.
Just my 2 cents, of course, and worth even less, no doubt.