What kind of water heater (make/model) do you have?
If it's a Suburban, are you are leaving the electric heating element switch turned on when there is no water in the tank, like when you drain the tank? If so, then you will burn out the element in a matter of mere seconds. The water heater MUST have water in the tank at all times the electric heating element is switched on/powered. It's explained here:
I'm kind of thinking that is what the tech was referring to when he told you not to leave it on all the time......but could be wrong.
If you have an Atwood brand water heater, page 4 of their manual says the same thing...that there must be water in the tank covering the heating element when the switch is turned on.