The water heater has several types of by-pass valves, depending on your RV. Depending on how the bypass is set up on your RV you may be able to drain the water heater without draining the water tank. Mine has 3 valves, one from the water tank to the heater, another from the heater to the hot water line and a third bypassing the water heater connecting the water tank directly to the hot water line. You will have to locate the valve(s) and see how yours is set up. BTW, my RV is 12 years old so yours may or may not be the same???