I have done floor work in many, many mobiles. I have always preferred to “fix them right” that is cut out the bad wood, repair or replace floor joists as needed & block around the edges of the patch so that the repair is solid. But I have nearly gotten over my head a time or two & wished that I had not opened the can of worms so to speak and tore up so much that I had to fix.
Having said all of that, if the floors have buckles or waves, then you may have to cut between, and parallell to the joists to let the particle board lay down flatter, to do an overlay, you may also have to shim in places where the board is rotten to bring it up level so that the plywood has a solid base.
I like to use screws to put the repair joists, block & put the plywood down
it is easier & better. I have used black drywall type as well as deck screws, but the square drive will work great but they are a bit pricey.
If done carefully & neatly, you can overlay successfully. Care taken to minimize gaps around walls & ducts will make the job better, & 1x4 or similar base will help hide the gaps. You may or may not want to caulk the edges, depending on where it is & if you will keep the unit.
There is one kind of floor that should be avoided, and that is a “honeycomb” floor. this floor consists of a paper honeycomb about 3" tall sandwiched between two layers of parcticle board, abd has NO FLOOR JOISTS. Save yourself time & money by setting the trailer and a few dozen c-notes on fire. Mostly these were late 60’s & early 70’s, I had a “Royal” doublewide (it was a royal pain).
Another type that presents problems & extra work is a trailer that has the floor joists running parallell to the frame, or the length of the box. This has the outside sill under the outside wall & no joists crossing the frame, only the steel out riggers every 4’. so if you have to replace any wood then there is nothing to fasten to along the outside wall, & you have to remove the old floor in sections & block in so that your floor has support on the outside wall.
I hope this helps,