chronic problems with small spaces, especially mobile ones:
hard to put enough mass in them to make a difference, hard to do radiant heat safely.
One option would be to get a stove designed specifically for small spaces - like a diesel boat heater if it's a diesel van, otherwise maybe something electric or propane - and then add thermal mass inside the van's insulation. Like jugs of
water or a big stockpot somewhat near the heater. This could hold quite a bit of heat (more than cob would, actually) but be easy to empty for travel. You don't want to boil the water in there though, unless it's a big open pot with some room under the lid to prevent spills. You don't want to evaporate too much water and make your space all damp. And you don't want to travel with the water full.
A safer option would be to
beef up the insulation, like making the whole van into a good sleeping bag. If you can conserve your own heat, with enough ventilation for safety, you will be a lot safer than trying to run a fire in there with you. A good dog or some company helps too.
-Erica