Cheap Heat systems are the best option for electric heating.
I used to swear by Vornado heaters until they seemed to have moved their production to China. Chinese junk is Chinese junk, so I find a heater that blows warm (not hot) air and is cool to the touch on the case.
With two of these, I leave one at 750 watts (RV wiring is generally substandard, so I don't want to risk full wattage), and the other gets powered by a heavy duty extension cord routed through the mouse hole, so I can use its full 1450-1500 watt capacity safely.
When I boondock, I use the heaters to get the trailer heated, then come 8:30-9, I power them (and the generator) off and let the RV furnace do its job.