Sounds like the particle board has gotten wet and now has to be replaced. It may have been some cheap thin stuff that didn't hold up. If if is just a few places you can cut out the bad spots or you may just replace all of it. For bad spots, use a circular saw set to the depth of the wood and cut on each side at the middle of a joist for a nailing surface and cut the other sides using a square.
Since your mobile home was built, the plywood manufacturers have cheated about a 1/16th" on the thickness so you will want to get some 15# roofing felt layered as a spacer. Replace with CDX plywood (some will use OSB) and screw with 1 5/8" screws that countersink.
Pull the carpet up to inspect it. I would consider replacing the whole floor as any leftover stuff may sag next year plus it is easier that patching. If linoleum is on it, then you will want to replace it all anyway. Remember that with mobile homes, they were built with all the floor decking laid down and then linoleum laid on top in the right places. Then the stud walls were put up. You may have to cut around stud walls and in some cases dig out the rotted particle board underneath the wall. Use a dremel type tool with a cutting disk to get underneath to cut the wood and nails and then slide the new stuff underneath.
Inspect the insulation underneath as it may have gotten wet and ruined. Also look at your plumbing, if you have some PE or PB with bad fittings, aluminum crimp rings, etc, replace that part of it while the floor is up. Good luck. Next time start a new post at the top so someone will see it.