We spend about a month up north with temps getting as low as 15 degrees, before we head to SW Florida.
In our FW we do the following every fall: place a ceramic heater in the basement and set it on 60 degrees. Put our heated water hose on. Cover the CG water service with a plastic 55 gallon barrel turned upside down with a heat light mounted inside, after flushing black tank we always put a gallon of RV pink in black tank (keeps black tank gate valve from freezing up).
We use 2 more ceramic heaters inside the FW, but we do run the furnace and use about 30 pounds of propane per week. With our make/model of FW the furnace actually has a small 2" duct that dumps from the furnace into the underbelly and keeps pipes and other things from freezing.