Bill,
If you are hooked up to city water, the most common problem is an overfilled fresh water tank with water running out the overflow/vent pipe. Cut off the city water supply and run the tank down with the pump, if that is the problem, the water leak will stop as soon as you run a few gallons out of the tank.
Assuming that you that you have verified that your leak is from an overfilled fresh water tank, there are two possible problems that can cause the condition. First the red hat valve can be leaking and allowing the city water supply to overfill the tank. Second, the fresh water pump internal check valve can leak, and let the city water supply run through it to overfill the tank.
Gerald