We camped in single-digit temps, with our ceramic heater as the primary heat, and the furnace set to about 60 as a back-up. Our furnace ran a couple of times; but I could hear the furnace in the 5er next door kick on about every 10 minutes!
Granted, we had Reflectix in the windows; vent pillows; A/C intake covered, etc. Our 5er does not have the 4-season insulation package, so our plumbing was winterized.
Our heater has a true "adjustable thermostat" - we can set it for 68 degrees, for example. It's nice because it won't ever make the RV too warm. And when it's oscillating, it can keep the entire living area nicely warm.