You can't fix the floor back to it's original condition, it will always be soft. When they build the trailer, they sandwich it together with glue then put a bag around it and compress it with vacuum. The best you can do is remove the rotted 1/4" wood and the 1 1/2" styrofoam and replace it, also putting in 2 X 2 supports. That will fix it so you can walk on it but it will still feel soft. Also, find the leak and fix it, probably under the tub/shower.
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