We had this problem on our Tiffin. Brand new pump, and fresh water tank would fill gradually when hooked to city water. Turns out, pump was fine, BUT, when you'd get around 50 psi, it'd leak past the check valve. Never had that problem on any other RV. We usually run 40-45 on the adjustable regulator, and 50 won't hurt any piping or fixtures in the RV, but the check valve didn't like it. Just my experience with a similar issue.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.