You've going to have to look at the piping that runs from the water filling port to the sinks, toilet, etc. You should find two valves, one in the hot water line and the other in the cold water line. They are there to drain those lines for winterizing and I suspect at least one of them is open. I think you can access these valves through a door on the side of the trailer.
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