Pretty simple really - from your description you have a Suburban DSI water heater, probably an SW6DE with the electric heating option. Unless the trailer manufacturer wired that electric heating element out to a remote switch located inside the trailer then it's obvious the switch which is located on the water heater itself is in the on position. The WH switch which you do have inside the trailer but which you say is off controls only the gas heating function. Lucky you that the WH tank has water in it, otherwise you'd instantly burn out that electric heating element. Assuming I've read between the lines correctly your water heater is working just as it should - electric heating element is switched on, gas heating is switched off.
