Once you open up damaged area, you may find where water came in. But like midnighshade says, first fix leak. Water can run down inside a wall and will show up as a floor problem.
When ever I have had to replace bad flooring in a RV trailer, I work from the top down. The membrane on the bottom of the trailer is usually a fiberglass reinforced waterproof material. Once you take up flooring you will probably see several joist that need to be replaced too. On one of my trailers I replaced about 20 SQ feet of floor and joist in the back corner under the tub, and on my neighbors trailer we replaced about 50 SQ feet of floor and joist in the front bedroom due to leaking storage doors. Here is how is was done: On one rain water came in at a damaged refrigerator vent and ran back under the tub in the adjacent bathroom. On the other storage doors for an under bed storage area leaked. You will need to take out any cabinets or items in the area of the damage. I ended up replacing floor joist, and some wall studs too. There are two kinds of wall construction, on some trailers the wall extends past the floor platform which make removing damaged floor easier. On others the wall sets on the floor plate, and the wood flooring has to be removed from under the wall, then to reinstall you need to jack up roof enough to lift wall so that you can get new floor material under the wall. If you need more information send me a PM
