Jane, I dealt with this a number of times. I would bet that your insulation first got messed up by leaking pipes. They will soak the underside of a trailer and the insulation. The water will move slowly through the bottom barrier but usually a lake forms in it pulling it loose from the joists in a big area and ruining the insulation. It has to be cut to drain and fix the leak and tear out the wet insulation. Then insulation has to be replaced so it also covers the plumbing which is below the joists. For small areas, I just stapled up a piece of plastic tarp to hold the insulation up. A large area is not easy for fun to do. I wouldn't replace the whole area with a tarp since it doesn't breath and you don't want a moisture/vapor barrier under the whole thing. However, tyvek may be the ticket.
Trailers usually were made with particle board and like Jay says, once it gets wet, it expands and crumbles. I have pulled up the old stuff and replaced it with 3/4" cdx plywood and have also overlayed the floor with 1/2" cdx depending upon the shape of the floor. If it has gotten so wet that it crumbles easily or you can run your hand through it, then replace it. It is a judgment call. Replacing is not easy but sometimes the best route. When built, they do the whole floor at once. Whereever vinyl went they laid down a sheet. "Then" they built the interior and exterior walls on top of it. Some walls are put on top of the carpet after it is laid. (they use a production line process) You end up having to cut and chisel under the walls to replace the wet stuff under the walls. No fun.
Particle board can get wet from leaks below that spray underneath as well as above the floor. I have also found that the metal siding leaks at the roof or windows, etc. and the water runs down the wall and soaks into the floor. You may not notice this until trying to replace the carpet or feeling a spongy spot when you walk on it. I fixed a plumbing leak in one last week in one and am now waiting a couple weeks for the mud hole to dry up before going back to fix the rest. It actually needs all the plumbing replaced first or it will happen again. Good luck.