We wasted no time getting to work on the repairs. The old structure had a 2x2 on the bottom, then the 3/4" particle board subfloor, and a 1x2 bottom plate sandwiched together under the studs. I decided to use a flat 2x4 on the bottom with a 2x2 bottom plate screwed up into the studs. The subfloor would no longer be part of the "sandwich" but would be screwed down into the exposed part of the 2x4. This would allow me to work around the plumbing with the new subfloor inside the trailer. You can see the dust on the ground that used to be the floor and bottom of the walls (completely rotted along the whole trailer). You can see I screwed a 2x4 into the studs and used a high-lift jack to support this section of the wall. I repaired each of the 4 wall sections separately.
Here I had to cut out the repaired section of the subfloor. It was 3/4" plywood and was much stronger than the rest of the floor.
These pics really sum it up. The floor has disintegrated and caused a gap in bottom of the wall.
All better now: