At the risk of being flamed unmercifully, I'm going to state a fact. Heat DOES NOT rise. The law of heat conduction (fourier's law) is the same as ohm's law for electrical conduction. Instead of voltage you have temperature difference. Hot air is less dense than cold air, so hot air will rise. But, if you heat a vertical pipe at the top, the bottom of the pipe will get hot.
Gus, I'm curious. Did you do anything to insulate the TOP of the bus (on the outside of the bus)? Does snow accumulate on top of the bus, or does it melt? If the snow accumulates on top, ignore this post! However, if the snow on top of the bus melts, you're losing heat. I suspect that duct taping some cheap bubble wrap to the top of the bus will insulate it well enough to allow snow to accumulate, which will insulate it much more.
The bus is warm compared to the outside air. It's a giant radiator. You've insulated the bottom. If the top were to be insulated, the effective area of the radiator will be reduced by at least 25%, which will reduce your heat usage by 25%. At least that's my theory. Of course it assumes no air is leaking anywhere, which is not the case.
I'm really enjoying your thread, it's quite educational as well as entertaining!