I believe a lot of RVs use a Dimplex electric fireplace--
http://www.dimplex.com/en/electric_fireplaces
These are not much more than a space heater, but people seem to like them for the appearance. They are limited by electrical service to about 1500 watts, same as most space heaters.
Be sure of what else is on the circuit that feeds the new fireplace, limitations as to amps available will soon let you know if you overload a circuit.
I prefer the storage available that a fireplace would take up; but then I have to take out/put up the space heaters when needed.
Joe