We have traveled in temperatures like this. Our furnace has a heat duct for the water bay. When driving, at -4F outside, generally the water bay can be as low as 38F. When parked, at -4F outside, generally the water bay is about 48F.
We had one trip, about -4F with strong side winds, and the water bay would get down to 32F after about 1/2 hour of driving. We would pull over for a few minutes, and the water bay temperature would quickly rise to the high 40s F.
So, we generally regard -4F as kind of the breaking point. If it gets down to -4F at night, it will usually warm up a bit when the sun gets up. So we can make it comfortably through another day. If it's going to be much colder than that, we can run the generator to power a couple of electric heaters, but usually we'll just dump the water, and winterize the system with RV antifreeze.
You have to know if and how the water bay is heated. We have a remote digital thermometer in the water bay, so we can monitor the temperature down there while we are driving down the road. Some coaches have a heat duct from the furnace to blow heat into the water bay. Other coaches have an electric heater. And some coaches have nothing. Some folks have found that hanging a 40 W light bulb in the water bay prevents freeze up. I don't know if that would be adequate for driving.
Regarding the LP furnace. While it is true that the heat is coming from propane, the furnace fan is electric, and we have learned that ours is a power hog. We run the generator for a couple of hours before going to bed, and by morning the generator needs to run again for a couple of hours to get the batteries charged up again. We have added extra house batteries. If you have a small bank of house batteries, you may need to run your generator more.
You have to know if your engine alternator is set up to charge your house batteries. Some coaches are, and some are not. If yours is not, then you may need to keep the generator running while you're driving.
Oh yes. Keep the fuel and propane tanks full.
Jim