I would suggest that it is the water pump check valve.
Easy test is to turn on your water pump, with no city connection, and pressurize the lines with all faucets off.... wait a while to see if the pump cycles (without turning any faucets on).
Even a small cycle of the pump will indicate it is the check valve.
However, this test won't work well if there is air in the lines.
Always use a pressure reducer at the water connection post....not at the RV inlet connection... I've ruined hoses because of too high of pressure.
Hope this helps
'07 WildCat 30 RLBS ~ West Coast Edition ~
Trail Air - Tri Glide Pin Box
2000 F350 DRW 7.3 Diesel Lariat 89k miles
Edge programmer, 4" SS exhaust turbo/back
Vancouver Island, B.C., Canada