question regarding electric element in water heater

[manorlife]

I tried searching but could not come up with the answer that I was looking for. In a boondocking situation where you fill up the water tank and presurize the system and turn on the heat element for hot water... I'm just wondering what happens if say you are taking a hot shower and you use up the last gallon of water in the system. will the electric heat element burn out since the water tank is now empty? or is there some sort of fail safe built in where it will not drain the last few gallons so the element is always submerged into water...