Idea -
If you're redoing all your plumbing w/ PEX, take the time to run the hot and cold lines together, and insulate them together... later if you want, a small
12V circulation pump can be used to keep the hot water running in a circuit - this will keep your trailer's plumbing from freezing when away from power w/o having to run the furnace, and you won't have to run the water for a while for it to get hot.
If you run the pipes below the floor, you'll need to aways use compressed air to purge them when winterizing, or put the drain valves underneath.
I'll leave the plumbing above the floor.
- Bart