Ruby-been-there done-that
What you are dealing with is a 2"x6" floor joist frame built like a stud wall @ 16" on center running front to rear and laying horizontal .(short way)
They are sitting on a metal frame running the length of the trailer, on top of these floor joist sits a cheep chip board that the adhesive disintegrates with the presence of moisture. The vertical stud walls are stapled on top of chip board sub floor.
Your water heater is probably electrical, If so find the circuit breaker and disable the source of electricity.
Your plumbing will be a copper line. After draining the heater using the valve located on the bottom of the heater, You can carefully heat the couplings with a propane torch and separate the copper lines or cut the vertical lines with a metal hacksaw at a convenient place to re-attache by re-soldering. Then remove.
You will then remove the damaged sub floor, just take a hammer to remove damaged areas. It will take a hammer and chisel to remove the material under the stud walls.
What your want to do to save labor and materials is, find the center of the floor joist, mark it squarely to the wall with a straight edge, and set a skill saw with an old or not valuable blade to 3/4'' depth and cut the damage out to the center of the floor joist, and remove the adhesive and chip board with a chisel.
If your lucky you can reuse the floor studs. If not take a screw gun and toenail in 2"x6" pieces where necessary to support the new 3/4" plywood sub floor repair. Then if you have an old water heater, I would recommend a replacement while you have it apart. Plan on at least 1 day to accomplish this task, don't forget to turn off the water supply before disconnect of the heater
Its not really that hard, but then I have been a professional floor installer for 34 years.
Good luck if you choose to do this project.
Paul T. Spurlock
Enhancing the image of concrete since 1975