I've been repairing mobile homes for 7 years or better...This is a tricky task if you don't have the tools to do it. You may get away with laying new flooring on top of old but you may have problems with the doors if they aren't cut up at the bottom. The best thing is to mark and cut out the bad flooring. The flooring under the walls is very difficult to replace, I wouldn't worry about it. When you cut up the flooring cut 6" wide splicers that will fit between the floor joists (you will use these to "seam" the old flooring and the new flooring together). Use screws, not nails, and apply PL200 to one side of the splice and then screw it to the bottom of the old flooring, running the screws from the top. When that is complete then run 2x4's, cut to fit in between each floor joist against the wall, for support, and attach those to the joists.. When this is complete measure and cut your new peice of plywood to fit the open area, apply PL200 to the exposed splicers and joists and screw it down.. That is the basics....But did you take care of what was leaking first? Check the caulking around the windows in the area or make sure none of your plumbing fittings aren't leaking or maybe, if you have a metal roof, your roof may need to be sealed.. Hope this helps you.