Although you can't use it inside the van (although I have been wondering about rigging up some kind of flue), I have a Ghillie Kettle (also known as Storm Kettle) which boils a litre of water in just a couple of minutes. There's no fuel costs as you use twigs, leaves, paper wrapping etc although I carry a bag of dry kindling in the van, and the worse the weather, the faster it burns. You can get a 1.5 L version, and I think a 2L version. I also carry charcoal so that I can use it as a mini bbq, and it's nice to sit around at night. One of the best bits of camping kit I've ever bought.