Whatever they used to build it the first time, it only lasted 11 years. If you're lucky, it wasn't built the way many travel trailers used to be built. They would cover the frame full width with a sheet of plywood or particle board then build up the sidewalls on top of that floor.
I'd suggest using pressure treated plywood to replace the original floor. Use foam insulation and perhaps a plastic sheet barrier between the foam and the plywood. Get as much of the rotted flooring out as you can, if any is left it will just continue to spread in the future. If some can't be removed, use a product like Git-Rot to treat any soft wood left in place.
Git-Rot is a watered down epoxy that will absorb into the soft wood and harden to stop any future water infiltration. I used to use it on wood boats, it does work for soft wood that you can't replace.