In case you're in a hurry I'll list the best sources for off grid and van life heat right here.
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While I don't recommend using propane for heat. In a budget situation a propane heater could be your best solution. However it costs half as much to burn fuel than propane, even less if you use small propane bottles.
The downside of using propane is both the cost and the fact that 4 molecules of water are produced for each molecule burned. This can cause condensation on windows and mold in your walls not to mention permanent damage.
This style of heater cost nearly two thousand dollars a unit until recently when the patent expired and Chinese versions were produced. Now you can get one under two hundred dollars. Diesel costs about half as much as propane to burn and the heat is dry heat like that of a wood stove. This unit comes with a remote control so you can operate it from a distance. I have one in the shed that I can fire up when I wake up from my van. That way it's warm when I go to the shed. Diesel heaters are very convenient and can cost less than burning wood. The downside could be refilling it with fuel but you can easily attach a much larger tank if desired or run in directly into your vehicle tank if you run fuel.
WOOD BURNING STOVE
One of the best sources of heat is wood. If the economy completely crashes you won't have much of a choice. Getting fuel could be difficult. They'd definitely security in that. The heat is dry and sitting around a fire is comforting. The downside is cutting wood is a lot of work, so is growing it. It's not the most convenient source but it's tried and true.
Personally, I have all three versions just to be safe but my favorite, so long as there's not an apocalypse is the diesel heater.
If you want to learn more about building an off grid homestead with next to nothing, read my book. I tell you everything I did to start my homestead with next to nothing for the first year.