You COULD have a leak, but the possible culprit might be the winterizing valve that bypasses the water heater. Are all the faucets closed? The system has to build pressure for the pump to shut off. Sometimes, I've had to run mine about a minute to get it to shut off. And, you're not leaking water through the drain valves under the trailer about halfway to the other end?
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew