I prefer not to let the inside of the RV get below freezing, as frozen condensation can do a fair amount of damage. In our soggy winter climate, condensation is a permanent issue.
I use one of those oil-filled radiators and a plug-in digital thermostat. The t/stat bottoms out at 45 degrees, which is higher than I really wanted, but I put it in the vicinity of the heater. I have an indoor/outdoor thermometer and I put the "outdoor" transmitter on the doghouse of the RV and take the display into the house. It has max/min readings, so I can keep tabs on how cold it gets inside the RV.
I run the power cord through a Kill-a-watt meter to see how many KWh the heater uses and in last year's fairly mild winter the total used in 4 months added up to around $35.