You need to bleed the air out of all the lines, all faucets hot and cold. Once the lines quit spitting and spatting air, and you have a steady stream of water, that that faucet off and go to the next one. The water heater will take longer, to replace 6 gallons of air with 6 gallons of water. If you turn the faucets all off before the tank is filled, the lines and the water tank will reach pressure and simply will not fill any longer.
Now, the electric heating element is on the bottom of the tank. As long as the heating element is covered with water, if you happen to flip the "on" switch, you're good, because the element is covered with water. But if the tank is empty, and the element is not covered, you'll burn it out in a matter of seconds.
Here's another hint. If your water heater has an outside rocker arm switch - on and off - always keep that "off" unless you are actually using the camper. Especially if you drain your water heater after every trip, make double sure that switch is off.
This way, if the switch for electric gets turned on inside the camper, nothing will come on. Because it's double switched and off AT the heater. It's REALLY too easy to hit the water heater button thinking it's a light switch, especially kids, if you are plugged into shore power, but don't have water in the tank. So, always make sure both switches are off unless you know there's water in the tank.
I installed a second switch on my water pump for the same reason. The switch for the water pump is right beside the main panel and the house lights. Several times, when parked at home, and the fresh water tank was empty, that switch would get flipped. The water pump would be running (for who knows how long) with no water in the fresh tank, no pressure ever built up to turn it off. After a few times of that happening, I installed a second switch. It's never been found running again or turned on by the "unknown ghost?"
So, check if the tank is double switched. By the way, it won't hurt anything if the gas comes on and it's empty. It will just get very, very, very hot. But nothing will get burned up if there's no water in the tank.
