Water pump - most likely. If you hook up to an external water source, it's no big deal, but if you are booddocking, the pump is how you get water pressure out of the tank. With the trailer power on (battery), do you hear a pump turn on in the water tank when you flip the switch? On our trailer, the water tank is under the bench seat. Is this bench seat in your trailer immediately adjacent to where the water inlet is on the outside of the trailer? We have one other switch in that location which turns on the igniter for the hot water heater.
