This information comes from Parny52. If you have anything to add, please e-mail me.

1. To replace part of a rotting floor, cut out the rotten area until you get to strong (good) plywood. Be sure you have part of one of the support angle irons under where you'll be patching.
2. Cut out a new piece of plywood to fit your opening, and fit it in from underneath the trailer.
3. Once it is in place, cut another piece of plywood and place it so half is over the new patch, and half is over the old (good) plywood.
4. Screw it in to both sides--this, along with the angle iron support of the trailer will support your patch.
5. Paint the patched area under the trailer with exterior paint.
6. Cover the patched area inside the trailer with new flooring.

And, pictures from pjeep's floor replacement