We "unintentionally" full-timed in our HC for 8 months after a PCS (military) move, while waiting for our house to sell. It wasn't really that bad. Temps in the Fort Campbell, KY area sometimes dip into the low teens in December and January. Here's what I found...
1. Temps below 32 require a heat taped fresh water hose and water source.
2. I could comfortably heat the entire coach with three 1500 watt space heaters I bought at Walmart, as long as the outside temp was above 25 degrees Fahrenheit. I also put a little compact ceramic heater in the area in the basement storage area, which made the floor above it MUCH warmer.
3. In temps below 25, I supplemented my space heaters with the propane furnace, which easily kept up with the demand. Plus, it kept my galley pipes from freezing. Yes, it was still chilly when sitting in the slides, but it was OK with a sweatshirt on.
4. A bit of heat tape Gorilla Taped to the bottom of the exposed drain pipes under the trailer will keep them from freezing. I turned off the valve on the gray tank when below 25.
So, bottom line, you can winter camp in a HC. However, it requires some preparation and diligence.
Oh, and electric was free where I was, so I have no idea how much I used. Probably a good bit with all the heaters and heat tape.