Here is a link to help size your heating system. Just select "Poor" insulation, input the size of your RV and the difference between the coldest exterior temperature expected and desired interior temperature. Push "calculate" and you get the total capacity of the heating system you need. Now subtract the amount you get from any other heaters you may have (if propane, use the BTU to Watt converter) (negate efficiency losses for now) and you are left with the amount of supplemental electrical heat you need.
Next you need to determine if your electrical service and circuits can handle this additional load. A portable electric heater uses (and generates) roughly 1500 watts (the same as a hair dryer). And you should only use one such heater per circuit (not outlet, but circuit breaker). So if you have two circuits available, that's roughly 3000 watts (or two hair dryers) of heat. Additionally, this example (two portable heaters) would require that you have a 30 amp service avaliable. More heaters would require a larger service and more circuits.
I hope this helps. Good luck.