When I replumbed my trailer I just left out the overpressure regulator and overflow valve.
I got a brass check valve with 1/2 pipe threads and that worked out fine. I also put one on the output of the water pump even though I was told the pump has one built in so you don't need one.
I just use the pressure regulator on the hose now and as long as I don't forget its fine
Actually my improved pluming can probably take normal pressures anyway. Turned out nice for me. Nice to get rid of the PO's hoses and clamp repairs
You can sort of see the check valve in this photo on the bottom left.
Good luck with your project.
There are more pics of my work on my website listed on the www button below. Look for plumbing problems.