Unless you have a 4-season camper or only see freezing for a couple of hrs per day then it might be a hopeless cause.
That section of water line that is way back, close to an un-insulated outside wall and hard to heat will freeze and shut you down. Your only choice to keep it warm is to run the heat and use a fan to blow the warm air back to the cold places, which will, in turn, more quickly cool the cabin, which will cause the heater to run more and pull down your propane and battery. you will need big tanks, and lots of batteries.
As others stated, figure out how to camp not using the on-board water system, rather use jugs of portable water. This means no showers, you will have to sponge bath in the shower stall with a pan of water or use a tree shower bag.
If you want to get creative and have a rigged system then bypass all the camper water lines, close them off, blow them out, and attach a hose of some sort to the water pump, and run that hose to the sink and or shower so you have it all inside. The big water volume will probably not freeze,and maybe easier to just keep the pump warm than a whole system. But this is stupid, the jugs of water are easier.
I was camping and the temp unexpectedly got to 11. The propane ran out in middle of night, and within a couple of hours the inside pipes froze. No damage, but they were frozen up and useless. I wished I had jugs of water then.
