Some roof top recreational vehicle air conditioners, for example Colman brand RV air conditioners have a heating element or ‘heat strip’ installed which is intended to be used as a supplement to the RV furnace. If you don’t have one installed it’s likey an option. The heating element is the equivalent in capacity to a small portable electric heater… about 1500 watts.

It’s generally used on to take the chill off the air rather the heat the RV. But you can use it when your furnace isn’t working to get you through a cold night. You may not be toasty warm but you wont freeze either.