The "right vehicle" could benefit from a smaller more portable version of todays outdoor wood burner. It likely would not be a 24/7 heat source unless one is parked somewhere where it would not affect anyone negatively.
I say "right vehicle" because some will have a good location or position to mount such a device, while most may not.
For simple discussion, lets just imagine a 6-8" diameter steel tube mounted vertically on the back of your vehicle... perhaps as simple as slid into your trailer hitch receiver tube. This tube has a cap that narrows like the tin mans hat down to perhaps only 2-3", where a 2-3" flexible tube exits vertically.
Inside the tube is an insulated layer wall, a firebox and a simple coil of tubing that wraps around and over that "firebox". Antifreeze is used in the tube and circulated to a heat exchanger inside the vehicle. The "burner" in hostile use environments would probably be clean burning plain old charcoal. The real wood type seems to last a long time.
In less hostile environments, wood or wood pellets could be used (when smoke may be less of a problem). With some engineering skills, it is possible to make such a tube to be fully insulated so that its only warm to the touch (except for the stack exit). It could also be made to "damper down" to make the fuel last longer, and in fact could also be designed to allow automated "stoking" by dropping in a new charcoal biscuit when necessary.
The fumes are outside, the dirt is outside, but the heat can be carried inside. Nothing stops a person from developing one that also could run on propane when wood or charcoal is not available.
Where to mount. Like I said, some vehicles could do this very well... like a school bus or narrow hooded truck. Rather than people thinking the air cleaner hanging on the side is actually an air cleaner,.... it could be a stealth outdoor furnace !
