You may find once you remove the bad flooring that you need to replace or repair some plumbing. While you have the floor open may be the best and easiest time to repair or even upgrade the existing plumbing. Before you start read Understanding and Installing Manufactured Housing Waterlines
If the floor damage runs under the tub see Replacing a Manufactured Home Bathtub
You should be able to to the entire job from the top side. Working under a manufactured home can be very challenging if not imposable. . You most likely wont need to worry about replacing the floor under the walls. In the event that you do not feel comfortable with whats is left supporting the wall, it is easier to remove the bad floor and any that is under the wall can be replace by sliding sections of plywood under the wall and tapping them in place with a hammer.
For the best instruction anyplace Take a look at Floor Repair Info Kit from Aberdeen Repair
This kit includes a 25 minute VHS video about repairing the floor in your bathroom underneath your toilet,and a booklet on floor repair.
This is not a high-dollar video - it is only $25.95 but features a real contractor clearly showing you how to do the job