First of all, there are many schools of thought in repairing a busted floor in a mobile home. If you own a pre 1980 model, it's likely it's going to have particleboard subfloors. Whether it's a pipe that breaks, kids run through the house like a bunch of drunken hooligans and before you know it, you have soft spots in your floor you find yourself playing hurdles over. I'm going to assume you have basic carpentry skills and some tools. You'll need a circular saw, reciprocating saw, a radial arm or chop saw if you can borrow one, a framing square, a good battery powered drill driver with an electric brake, 2 inch and possibly 4 or 5 inch deck screws, some glue, a tape measure, gloves, a pencil, as well as some 3/4 plywood or OSB, some 2x4s and/or 2x6s.
I'm going to assume you're going to have to cut out the damaged sections. If water was involved, you have to remove them. If not, you can just cover them over with new subfloor. If you do this you will need longer screws and pay attention to where the joists are. My advice is to remove the damaged sections even if you are planning to just cover it over. If there are any water lines that look questionable, fix them now too. I use Shark Bites in place of the PEX connecters. They're expensive, but they work. Remember, this is dangerous, dirty and tiring work. Be very cognizant where you're cutting, screwing together wood, or even gluing as these can cause serious injury up to and including death as well as property damage. Do not use power tools if you are tired, high, drunk, stupid or any and all of the above. I have no control over the quality of your work, so you alone take the credit or responsibility for same. Do not work on moldy or mildewed floors as these should be removed by a professional versed in mold remediation. This work could exceed the value of your mobile home. Do not assume this fix is or that these are a complete or comprehensive set of instructions. You have to adapt to fit your situation and when in doubt, call a professional.
Use your circular saw set to a depth of your subfloor only to score the damaged parts. Do not use a reciprocating saw as you may hit wires, gas or water lines. Make your holes as square as possible as it will save you hours of frustration trying to fit patches. Sister up the joists with the 2x4s to give something to screw your new subfloor into. This means using screws to secure the 2x4s to the joists or 2x6s if you want. You'll also want to use 2x4's perpendicular to the joists to attach the new subfloor to the old with screws. Just screw them from the top with the lumber underneath the edges of the old floor. Be careful that you use screws that are long enough to secure them, but not so long they go through your hand while trying to hold it underneath same (OUCH!). Then just measure your patch and secure it with more screws, using one every 6 or 7 inches and construction adhesive if desired. Replace the floor covering with one of your choice. Maranatha!