I had a brother who camped in my front yard for a year he used a tent (really small ) wood stove to heat a 35 foot camper and it was at times 90 degrees in there. he ran the pipe through the roof vent. to heat a bus you will need a very small stove.
although I agree that o2 and open flames is not a good idea.
2010 ford e350 superduty v8 7 passenger bus