We have a small quartz 1500w electric heater that we use in our 26ft motorhome. It will work fine on full power, but we only use 1/2 power (750w) unless it is so cold that full power is needed. We use a regular extension cord (think it is rated at 20amps) to plug in. We also have a power meter so we can monitor voltage in the RV (making sure there is not to much drop in the extension cord). The shorter the cord and the heavier the cord the better. Oh, we turn off the heater if we need to run the microwave or electric water heater.
Have never had a problem. We also open cupboard doors in appropriate places to allow warm air to reach water pipes and tanks.
However, if the temps are getting down into the 20's it may struggle to keep the temps in the comfort zone...depending how large your RV is. May need to augment with the RV furnace during periods when the heater can't maintain temperature. The RV furnace also circulates warm air to pipes. Recommend some RV antifreeze in the holding tanks if they are not winterized.
In the Winter we set the electric heater thermostat to keep the MH at about 40 degrees. I can monitor the MH temps from the house. Ops, need to go turn on the heater....its 37 degrees in the MH right now.