If your fresh tank is filling , when using park water supply and turning the fresh tank fill valve doesn't stop the flow; there's a good chance the valve is defective and you'll have to replace it.
There is also the possibility of a defective back flow valve in the 12 volt water pump , that's a second route that city water can enter your fresh tank .
Does the fresh tank fill quickly or take a while ?
Quickly , probably the fill valve .
If it takes hours then the back flow in the pump is the more likely.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.