Well, if the stove has carbon scrubbers and a way of catching the sulfur and other by-products of burning coal, then great - but otherwise, it's the dirtiest (albeit cheapest) fuel you can burn. Think of the legendary environmental health effects of burning coal to heat residences, in addition to the widespread blame for acid rain placed on combustion of coal. And then there's the coal mining itself, which would be bad enough if it was just miners getting black lung; now that mountaintop removal is the norm for coal mining in Appalachia, the impacts include obliteration of existing ecosystems and posing long-term risks to communities downstream of sludge impound ponds.
So no, I won't be burning coal to heat my bus if I can help it. That said, you're welcome to make any decision you like. It's just a matter of balancing the pros and cons for each of us, and I feel that the cons of burning coal outweigh the pros in my circumstance.
That said, what do you use to heat your bus, Nat? I've been trying to track down a definitive statement from you on this matter, since you seem to know what you're talking about, but I haven't found anywhere you say what you use.