A propane fired furnace still needs electric to run the fan, it will run on the battery ,but will kill a single battery overnight, if you set at much above 60, it may even drain 2 batteries, depending on state of charge when you start.
I can see this being true with some combinations of batteries and furnaces-especially on larger rigs with small batteries. But it is not universally true. My 24' has 3.5 amp furnace draw when running. Even in low to mid 20s temp the furnace duty cycle is about 50% for low 60s sleeping temperatures. If I was having to run generator middle of night to provide heat (other than emergency situation), I'd be at the drawing board...