I had the same thing happen last June and went right to the forum for help. Thanks to fonzie for the following help and much like Flybob's. Worked great and have had no more problems.
"Your check valve internal to water pump is leaking. Disconnect city water, turn pump on open cold faucet for one minute. Stop water pump (turn off) reconnect city water and hopefully it sealed. If still leaking try a few more times. If it does not fix, then either new pump or check valve kit or add check valve to output line of water pump....."
check valve in water pump system
Heartland Big Country 3150RL
2013 Ford SD F250 4x4 SC; 6.2
Reese 18K Elite w/slider - Rockford, Michigan