Since we live in paradise (AKA Alaska) it is below freezing about half the year or more. In some instances we have had to run three furnaces, the vehicle heater and some electric space heaters to keep the cabin warm (like when it was 30 below). Life is risky. There are many risks traveling in an RV, most greater than the slight increase in risk associated with running a furnace while driving.
When we bought our 1999 Bounder the factory rep told us they had been tested to 8 above. This is a flat out lie. The supply line from the water tank is in the coldest location possible and without heat it freezes at +25f. For winter travel and camping we do several things:
1. Fill the water tank with hot or very warm water.
2. We put a piece of insulation foam inside the bay doors to hold heat and delay cooing and freezing.
3. We have extension cords running from the 120 bay outlet with drop-lights placed beside water pump, under the water tank by the shut off- drain valves, and wherever water lines are exposed to cold in compartments.
4. Traveling in temperature at or below 0f in many RV's will require that the generator and the furnaces are run continuously (which is probably good for the generator since most fail from lack of use). We put more hours on the generator than on the engine. Of course this is largely due to the fact that the last time we camped in a campground with hookups was three years ago.
5. In cold weather (below freezing all day) it is a good idea to fill the water tank with warm water and run the warm water through all the lines (turn on each faucet until it comes out warm - both hot and cold). It is also a good idea to use the water from time to time in case there is a cold spot in the system.
6. Because we use the RV year round we do not put anti-freeze in the water lines. After every trip we blow the lines out (and keep the lights on until we do if we get home late).
If the bay doors have 1 1/2 to 2" foam behind them and foam, newspapers, cardboard, etc., on the floor one or two 40 - 100 watt light bulbs (incandescent, not LED's or CFL's) keep the compartments pretty warm. To start out you can monitor compartment temperature wit a wireless indoor/outdoor thermometer - just put the outside sensor in the compartment. You may only be able to monitor one compartment at a time because many different thermometers seem to use the same frequency to send the readings.
7. I am going to test battery warmers to see if they can be placed on the water tank to keep it warm. They might be an alternative to light bulbs.
8. Many problems are caused by unused space between the ears. Always have a backup plan and (backup systems). I carry electric heaters in case the furnace quits or it is too cold for the furnace or light bulbs alone. I carry indoor safe catalytic propane heaters in case the generator quits. If something happens and the water system can't be kept warm, drain it. If you can't blow it out open the drain valves (all of them) and drive around for a while. If you are not in Kansas (or someplace similar) go up and down some hills and do a few stops and starts to get the water out of the system. If all else fails having a kit installed on the water system that can be used to put anti-freeze in the lines is preferable to freezing up. If you don't like the taste of RV anti-freeze (propylene glycol is not supposed to be poisonous) you can always pump in Everclear or your choice of 100 - 150 proof beverage. Come to think of it...maybe the best thing is to do that in the first place and forget everything else I have written.