Because RV travel trailers are often kept in areas of high humidity, and often used where there is a lot of atmospheric salinity, they are prone to moisture and salt damage. The interiors are particularly vulnerable to the moisture, and just a single soaking can cause disintegration of the particle board used to build floors. It is this far more than rotting that can cause floors to feel soft underfoot. If ponding has occurred even once, from a plumbing leak or from rain getting inside, it is likely that a complete replacement of the floor in that area will be necessary. Prompt repair is vital in keeping your camper serviceable.
Move around the whole interior of your RV travel trailer, testing the floors everywhere for a soft feeling and noting the locations on a roughed-out floor plan. Be especially vigilant around slide-outs, places where plumbing is above the floor such as in kitchens and bathrooms, and all around your freshwater storage tank if it is inside.
Lift away your floor coverings above the soft areas. Sometimes you can get a wide pry bar under a corner of carpet, as long as it is thoroughly dried out, and roll it backward while moving the pry bar backward and forward to break the glue. The glue will probably have been softened by the water. Floor tiles and linoleum are usually impossible to lift out in reusable pieces.
Use your permanent marker pen to make a wavy line that follows the outside of the area of floor that feels soft, which is often also swollen. Use your stud locator to find the nearest underfloor support in all four directions, then make four lines that mark out a square box around the wavy line. Draw another set of four lines 1 inch outside the first box; this second box is your cutting line.
Check thoroughly for screws, nails and any other metal in the wood along the cutting line. If you find any they must be removed, because they can become dangerous flying projectiles if you catch them with your cutting tool. They will also blunt your blade.
Drill a hole into the floor where there are no underfloor supports, and measure the thickness of the particle board. Adjust the blade height of your cutting tool to that measurement. You do this by adjusting the blade shroud if you are using a circular saw, by adjusting the head depth if you are using a jigsaw or by adjusting the blade guard if you are using a reciprocating saw.
Follow your cutting lines exactly. The replacement panel will make a much neater fit if all the cut lines are straight and all the corners are perfect right angles. Lift out the square of soft flooring and you will see that a 1-inch lip of underfloor support has been created around the hole. Use your tape measure to learn the precise thickness of the sound floor around the edges and make sure you buy replacement material that is exactly the same.
Use marine-grade plywood for the replacement panel to prevent any future water that may get in from doing the same amount of damage again. Measure your hole, transfer the measurements to the ply and cut it once more making dead straight lines and perfectly square corners. Before you put it in, check the underfloor insulation and replace it with a comparable insulator if the moisture has made it discoloured or deflated.
Drop your new panel into place. It should fit without any horizontal movement in any direction and seat firmly on the 1-inch lips of the four exposed underfloor supports. Drill a countersunk pilot hole every 6 inches and secure the ply panel in place using treated exterior decking screws. Lay new floor covering according the manufacturer's instructions.
Install a carpet threshold bar over the join between your new floor covering and the RV travel trailer's original floor coverings. This will disguise the work in an attractive way.
Always wear appropriate protective clothing when working with power tools.