I use one 1500 watt heater in my 24' class C. I generally use it in hookup RV parks when it is cold, and supplement it with the furnace when it is very cold.
I've used it into the mid or lower 20s and it will maintain high 50s-60 inside overnight. For me perfect for sleeping. In the evening as temp drops it usually is maintaining 70 plus in the RV. I use the furnace additionally for morning warmup or for other times when the electric heater can't cut it.
Couple things to watch out for- I agree check the plug and outlet for excessive heat and monitor the heater for proper operation. Some people replace the outlet with a better quality outlet.
Also some RV furnaces blow air into bottom compartments, plumbing chases etc. to help avoid freezing. If yours does take that into consideration when operating electric heater in cold conditions.