I have the same issue .
Depending on the brand of pump there maybe a rebuild kit available for the pump head ( that contains the back flow valve ) or the pump may have to be replaced .
Alternative is to install a shut off valve between the city water hook up and the pump.
Spartan would have no info on plumbing for your coach ; Fleetwood would be your only possible source for info.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.