If you will have at least a 30 amp AC service, perhaps you could go with electric heat. Two or three of these might do the trick:
http://www.walmart.com/ip/World-Marketi ... r/13141908
I use one of these to keep the interior of my RV from freezing in the winter. I just set it to the lowest power setting. An oil filled space heater is probably safer than one with an exposed coil. Price is right, too.