City Water In Filling Fresh Water Tank

There's a check vavle in your water pump that keeps city water pressure from flowing backwards through you pump. It probably has debris in it. The filters you can add before the pump help but wont completley alleviate the problem for some reason.
Take the head of the pump apart (very easy) and simply rinse all the parts off under water. Re-assemble and it wil be fine.
I've had to do this several times, especially when an RV was new.