The best source of heat in a cargo van? An Espar heater, hands down. Nothing else even comes close in efficiency or cost effectiveness. Like Cheri said, insulate the van like crazy. It helps with keeping warm in the winter, as well as keeping cool in the summer. An arctic sleeping bag, several layers of blankets and clothing, yeah, well, keep those on hand maybe in case of an emergency, but you won't need all that if you have an Espar heater. I sleep nekkid on top of the sleeping bag, and I sleep quite comfortably. Most important, my (usually sockless while sleeping) toes sleep comfortably. If my toes are cold, I am not a happy camper. Not at all.