What's the objective in circulating the air? To overcome stratification, so that your toes get some of the warmth rather than it accumulating at/over your head? (need fans to push air from ceiling down to floor.) Or is the heated air localized to one area, and you want to share it around the length of the bus better? (need ducts with fans drawing air in where it's warm, probably above the heater, and discharging where it's cool, probably down low at the far end(s) of the bus.)
Just moving air from the ceiling in a warm zone to the ceiling in a cool zone could help, but I suspect it might be even better if the discharge is nearer to the floor instead of the ceiling.