We really need more information. Some of the answers by others above may or may not work depending on how you answer the following questions.
Electric space heaters do not heat your wetbay. Opening cabinet doors does allow more heat to plumbing behind them.
What is the reason or purpose for the 50 degree setting? What is your goal?
Have you winterized? How did you winterize?
How are you storing the fridge? What kind do you have?
If you use the propane heat, you will need to check it once in a while so you can take it to refill if it gets low. A propane furnace does not operate off of 110v from the house. That is not to say you do not need 110v. Here is why, working backwards. Your control boards and igniter are powered by 12v supplied by you battery(s). Your battery is charged by a converter or inverter/charger, which turns 110v into 12v. The converter or inverter/charger is powered by the 110v. Without the 110v, a single battery would die in about 8 hours. Some sooner than that.
So now, back to the questions above.