A simple lit candle will prevent the inside of your van from dropping below freezing. Just make sure it's on a solid base, maybe in bowl. The seals around the doors and the floor aren't perfect so the amount of air coming into the van will ensure the candle remains lit.
Before any of you start screaming that this won't work, my Dad is a retired plumber and that's how they used to prevent pipes within complete roof voids from freezing.