The floor is made from 3/4" particle board, but will be replaced with
3/4" plywood. (treated plywood by the door, which is where the floors
always go bad in these trailers).
Why settle for ordinary pressure treated plywood when you can use PWF
plywood for the entrance?
Pressure treated plywood can't hurt, but it would be smarter to fix the
leaky door situation. Adding a storm door to the exterior helps a lot when
you don't have any roof overhang (typical on mobile homes). If you have to
replace the door frame, install a Jamsill pan to direct any water to the
outside of the building.
If you're going to be removing rotted particle board under the exterior
walls, then I'd recommend you break the length of the job into 4 foot
long sections, and do every 2nd section at a time. That way, the
sections that are still in place will support that exterior wall while
you're removing any rotted particle board from under it.
I suspect the original poster is doing more of a "patch" job, cutting up to
the wall framing, but leaving the strip of particle board under the wall
plates. That's the approach I used with our old mobile home because of the
vinyl covered sheetrock. It's not the best approach, but most mobile homes
don't have the best construction anyway.