I've seen a number of threads where people have the city water supply turned on and their fresh water tank is filling up.
My pump started leaking by as well and would fill the 60 gallon fresh water tank in 24 hours or less, so I figured I'd post some pictures of what I did to fix it.
The first thing I checked was the intake screen. I thought perhaps debris from the tank might have gotten into the pump and blocked the check valve. The intake screen was clean as a whistle.
I removed the pump and right away discovered a couple things:
1. there is a red wire connected to a white with blue stripe wire. When I touched it, the wires fell out of the crimp. So that was a problem waiting to happen.
2. the two red wires on the pump have connectors so they can be disconnected and the pump removed for service. However, the black ground wire did not have a connector - it was hard wired to the camper wiring.
So, I removed the eight screws on the front and opened the unit up. I inspected and wiped down all the diaphragms. Nothing seemed amiss, so I then removed the check valve. That's when I found a little piece of something (don't know what, grass perhaps) on the seal.
Removed that, put everything back together and problem solved.
Here's the pix: