Sounds like you are leaving the pump on when hooked to city water which won't hurt the pump but if you use a pressure regulator that is less than the pump pressure it will use the tank water first. I have an old Jayco designer and the pump is very quiet.
One thing I just thought about, on our Jayco we have a single hose hook up and a 5 valve water control panel. If memory serves, one of the valves selects between the tank and city water. If you have the same valve set up, is the select valve set half way between the two?
Edit: I went back and read your 1st post and is sounds like you have a similar control panel to our camper. The valve in question selects between "city" and "normal" which should be the tank. If it is set to city it should not pull water from the tank even if the pump is left on.