You didn't mention if you have to pay for electric. If you do not, using space heaters should help keep the rig warm. Just make sure you watch which circuits you're putting them on or you'll run into tripping your breakers. IF you ARE paying for electric, then the cost per BTU of propane is usually lower than electric. If you will be stationary for a long time, check with the local propane supplier. It might be worth checking into renting a 100kb tank and having them come fill it as needed. Usually the propane service is the chepest per gallon in any area.
