My electric heating element burned up many years ago when our rig was a few years old. I checked the heating element using a volt meter.
After I completely drained the hot water tank from the outside with our rig on our flat driveway, I unscrewed and removed the element, but water tried to rush out. I quickly reinstalled it and got lots of old towels. When removed the water kept coming and coming, the towels could not contain so much water..
I called Atwood about it. The service tech told me 1/2 gallon of water stays inside to protect the electric heating element in case someone accidentally left it on while plugged in. He also said the design is such that freezing temps will not harm anything.