The water heater will not be a problem if it is on. Perhaps if you have an outside tap and pump switch, you can just turn off the pump outside, and relieve the water pressure through the outside tap. This should relieve both hot and cold,(correct me if Im wrong someone) water pressure. From what I understand, many pipes that burst, do so because the water in the line increases the pressure as it freezes. If there is no pressure in the lines, nothing should increase enough to hurt the pipes. I agree that temps at night near freezing generally isn't enough to freeze the pipes, unless you have several continuous days below freezing. By relieving the pressure from the outside, you do not have to risk going into the camper, and possibly have cross contamination. This kind of defeats the purpose of isolation. Your wife can just turn the pump back on inside, and its ready to go. She can also just turn off the pump herself, and relieve the pressure on the inside taps. Good luck, I hope this helps.
