One more REAL IMPORTANT piece of advice. If you have the water heater bypassed, when you turn on a hot water tap inside the RV, you will get water out of it.........since the cold water line is actually tied into the hot water line, when opening the bypass valve that connects the two lines.
So, if you open the hot water tap and then think that because water is flowing out the hot water tap, there must be water in the hot water heater itself, you'd be incorrect. Remember, the water heater has been bypassed.
Now, if you turn on the electric heating element to the water heater (thinking there is water in the heater), it will burn itself out in a mere matter of seconds.
You need to make sure the hot/cold valves are turned correct for usage, the bypass valve is closed...and there is water inside the water heater. You can usually check for this by flipping up the pressure relief lever on the outside face of the water heater.
EDIT: Fonzie gave the short version and beat me to it. LOL