I've received a lot of questions regarding how to go about replacing flooring in mobile homes. Usually, most of the floors in mobile homes are made out of particle board, which usually wears down and breaks easily over time. In most cases, it's better to have a more solid type of wood as your flooring such as plywood.
As for replacing flooring in mobile homes, if there are only a few soft spots in the floor - you don't have to replace the whole thing. What you can do is pinpoint the soft spots by cutting out the damaged area(s) and replacing them with plywood - this is what's referred to as subfloor repair.
To help see how this is done, check out this video.
(If you're interested in reading and learning more about mobile home repair, I highly recommend these books).