This is the setup that was under the streetside bunk at the far aft end of the trailer. The PO had tapped into the external copper with one of those quick taps. It was broken off. I have no clue what he would have used that for. He had also put on a standard residential hose bib as a cutoff valve on the outside.
The photo above shows the pressure regulator, a pressure relief valve, and a drain valve. The latter two drain through the floor of the trailer.
The following shows the relocated assembly. I cut the hose bib outside the trailer and cut the copper a few inches inboard of the pressure valve assembly. I mounted the assembly on the plywood that once supported the Univolt, under the stove. I pulled up all of the copper supply line from the aft of the trailer to the pantry beside the frig, and the water hose splice from there going under the floor. I sweated the copper pipes together under the stove and re-routed a minor bit or two. I sweated up a few pieces of 1/2" copper to extend through the floor to the outside just aft of the battery compartment door. Sorry I didn't get a pic of that coming out the trailer yet. It just sticks out of the bottom of the trailer with a female pipe thread fitting sweated onto it, and an adapter to screw on the water hose.
It took me most of the day to accomplish it, part of which was figuring out the routing of the drain tubes (not shown). After this photo I added some fasteners to secure the assembly to the plywood. I think the hose bib in this photo is something the PO installed to add antifreeze to the system, or maybe to assist in draining. Does that sound likely?
My new work showed no leaks on a pressure test, so I was very satisfied with the results.