First, I had to finish waterproofing the dressing room. I had originally used Silicone caulking to seal the gap between the new wood over the rusted out mangers but it started to decay and pull away, letting water in.
After much research I turned to marine caulking. It was recommended by several people that put living quarters in horse trailers so good enough for me!
Next, I dealt with a roof leak in the horse area and in the dressing room by using Eternabond tape. It's a great tape like thing that provides a permanent waterproof bond. It's used on the roofs of RVs all over and works great on horse trailers. It's very sticky and does not come off once down without difficulty, so use caution and apply straight!
I painted the inside of the dressing room with Rustoleum paint. It's an oil based paint (you need mineral spirits to remove it), sticks to metal, and prevents rust from coming through the paint as well as stops rust from growing.
I installed a foam puzzle mat over the plywood floor. It made the trailer look nicer, and provided cushion and some insulation
Here's the dressing room before, you can see the rotted wood in the corner. I tried to remove this wood but it's bolted onto the outside of the trailer and a panel on the outside actually came loose. I think I'd have to remove the floor and drop it out the bottom to get it out without damaging the walls so I chose to leave it in and just cover it up.
The other side of the dressing room. You can see where rust from previous leaks came through.
Here's the puzzle floor going in. It was simple to instal although I did struggle in the rounded areas cutting it to fit right.
Another picture of the paint and floor. Nice and clean and fresh.
Next, I'd like to cover up the bottom of the mangers and replace the roof vent which leaks. For now I have a bucket over the vent when the trailer is sitting and that has stopped the water from coming in.