water connection problem

my bounder had a small brass valve that had to be closed/opened to direct the water flow. the water pump would always periodically kick on as it was losing pressure. it had an O ring that had a tiny nick in it and would let water seep by to reduce pressure and cause the pump to go back on.
bumpy

2 possible reasons

One is the on-board pump internal check valve is leaking allowing water to backflow thru pump into fresh water tank

The other way is if you have a water panel with valve/switch that you use to 'fill' fresh water tank, it is not fully closed or is leaking thru.

So if you 'gravity' fill your fresh water tank....pump check valve.

Sometimes using pump for a day or so can clean up the check valve (flapper) and then allow it to close.
Sometimes you can disassemble pump, clean up check valve and it will work just fine
Sometimes it's just easier to install an in-line check valve in pump suction line (about $10)

OP did say he has a Cougar 5er. Yes you need to look at your city water inlet. Does it have a valve that says fresh water tank fill/city water/normal? (or something along those lines). If so, that is where the water is coming from. If not, then that "before mentioned" check valve is likely bad.

You didnt mention if your 5er is brand new. If so, then there is an outside chance that your unit is plumbed wrong. (Yes the PDI should have caught that) but it is possible if new. Very unlikely, but possible.

You do not say what kind of rig you have.
My MH has a valve to fill the tank while I am hooked up to city water.
Could yours have he same and the valve not be closed?

turn your pump on and back off see if it stops.If not there is a check valve on he line that has went bad