I know what you mean, about mobile home floors. I have helped my brother with his, when the water heater leaked into the bathroom, and bedroom. Glad I live in an old house.
Yeah and Particle board isn't any better. Once it gets wet the glue breaks down and your left with a spongey mess. I had to replace everything around the flange. I found a support bracket that let me put the new floor (plywood!!! yeah!!!) in a single oddly shaped piece cut from one 4'x8' sheet. Used my noggin' on that one and made a paper template. Next up is putting down the new linoleum, installing the new toilet, and then putting base board back up. And after that I get to do something really fun, and that's use my new sledge hammer to destroy the old shower stall Of course my wife expects me to build a new one after that but hey, no plan is perfect.