Most Class A motor homes. if occupied with the furnace oprating are good to around 20 degrees safely.. Tonight in NC that may be an issue. In the Mountains of SC, likewise, where I am. 20's anticipated so I should be cool. and I do mean cool, both meanings. Been in the 20's many times.. Been colder a couple times.
If you have no water in the system you are safe
For added protection a few 100 Watt Rough Service light bulbs can be scattered about.. Make sure they are caged, Wither a plastic or wire cage and not touching anything air free to circulate around them (They do get hot) I have a string of C-9 Lamps under my fresh tank, about 270 watts total (each lamp 9 watts 30 lamps) and a 100 Watt RS Cagged in the valve bay. Together they give me an additional 8-10 degrees.