Fresh Water Tank

It is likely a faulty check valve in the ShurFlo water pump. In effect, water leaks past the pump into the fresh water tank. There's supposed to be a check valve in the pump to prevent this but it can fail due to debris not cleaned out in the lines after assembly of your 172 and messing up the check valve or even have heard of the check valve being installed backwards.  Before pulling apart you could try running the pump for 5 to 10 minutes and see if anything will dislodge from the valve; some have had success with that method.  If not then you will have to disassemble and hopefully find the debris that is keeping the check valve from closing or maybe you will find it was installed backwards.  Has this been an ongoing problem?  I have to admit, our 177 is going on year 8 and I don't know if mine works correctly because I have never hooked up to municipal or other water supply, we always carry our own well water with us as we normally dry camp.