Make your stovepipe taller. When you have a couple elbows in it, you add resistance to the heat rising out of the pipe...the example I'll give is like installing the exhaust pipe on a clothes dryer.
When I install a dryer for folks, the instructions state to keep the runs to a minimum, and that every 90 degree bend is the equivalent of adding ten (10) FEET to the exhaust.
Plus you may have one more factor:
when wind blows over a bus, it can add or reduce air pressure over the bus. That in itself can cause backdrafting problems. A taller stovepipe gets the top of the pipe above this downdraft, and helps solve the problem.
That's why building codes evolved about chimney heights over house roofs, to reduce or eliminate the backdrafting problem from house-induced downdrafts.
Hope this helps.....
The tool storage is nice, but where do I put the bed?