I used to use a two element heater in our Arctic Fox and toy hauler. one element was 500W and the other was 1000W. Ours allows you to run either element or both at the same time based on how cold it is and how much power you have available. At 500W the heater would run longer but the camper could be powered off a standard 15A circuit without fear that you you were going to pop it if you used the microwave or convection oven. My current Host has an electric fireplace that has taken the place of my space heater.
If you have ducted heat going to holding tanks, the space heater will not push heat in to that area. You will need to wire in a bypass switch for the fans or run the electric heater along with the LPG furnace when the temperatures get really cold.