I came across this water pressure regulator. It looks like it would work great. Preset to 40 psi and made of brass and stainless steel. Attaches to the faucet or one could attach it to the hose connection on the trailer.
It's not cheap $57.75 but could save your system if the internal one fails.
It is sold by Gemplers; the part #192109.
Just click on the picture, it will take you to their web site.