camperpaul is on the right page. not by choice, but i have camped when temps were in the 10 to 20 below range. i put 100 watt bulbs in a safe place by the water pump, the back of the water heater, in the water fill area, and the dump valve compartment. i know that may be overkill, but i believe i did it right, because i didnt freeze up.
there is no way you can keep a coach comfortably warm at these lower temps, even with an electric heater. we did keep it about 50 degrees inside. we used an electric blanket on our bed. what a life saver. my wife was ready to string me up for bringing her to such a cold place.
i travel in my work and was in the gas/oil fields in wyoming at the time.
there were many others in the campground that used the same method of protection.