Might be cheap from the standpoint of hookup to an un-metered (and therefore "free") electrical connection in a CG, but no way is electrical resistance heating more efficient than LP or natural gas. Either provides more heat when burned per unit volume consumed than a kilowatt of electrical energy.
Cheap heat could easily be had or improved at RV factories by better insulating and sealing rigs within the confines of their construction (wall/roof/"basement" 'thickness') and I don't mean using the mylar bubble wrap crap.