[RMcNeal]

Your tank heaters are 12vdc and do not use propane. They are thermostatically controlled and will only turn on when the ambient temp gets too low (can't remember the temperature, but near freezing).
If you have a heat pump, you can reliably heat the RV down to the low 30's with some supplemental heat such as the fireplace and a small ceramic heater. If not heat pump, you will need to run the furnace which does use propane.
I've stayed in single digit temps while hunting for several weeks in the mountains of WV. Very seldom use the furnace except to break the chill in the early morning hours. The combination of the heat pump, fireplace and 2 small ceramic heaters can keep us in the mid 60's inside. I also purchased a wireless temperature sensor with 3 remote sensors and put one in our rear under belly, one in the front basement passthrough and one outside along with the head unit which stays inside. Underbelly and basement typically stay in the low 50's when outside temps are in the teens, so no real worry of freezing pipes or tanks.
As stated, fill your fresh water tank and drain and stow your hose so it does not freeze on below 32 degree days. If you expect below freezing temps for extended time periods, it doesn't hurt to turn on the tank heaters as they only get warm like a heating pad. Just enough warmth to prevent freezing in the tank. They do use a lot of juice, so if you are boondocking, use sparingly only when necessary.