The anode rod. Looks like a large nut (I believe 1 1/16") at the bottom of the water heater tank, stand off to the side, unscrew and pull the anode rod. And I say stand to the side, water will come out a good three feet.
Also, that anode rod can fly out so be careful.
When winterizing the trailer in the winter, drain the hot water heater tank, then turn the valves to bypass the tank and then add the anti-freeze. That way you are not winterizing the hot water tank and using a bunch of anti freeze. If the tank is drained no hard will come to it.