Just make sure you keep all the cabinet doors open inside to let the heat get to the pipes. I don't know how your camper is set up, but heating the inside will probably not help the black tank, unless you have a black tank heater or a lot of insulation.
And yes, the furnace can go through a lot of propane. Can you run an extension cord out to the camper and plug in a small electric heater? A friend of mine did that with his small TT when a friend of his needed a temporary place in the winter... he said it seemed to be less expensive than propane and worked fine.