Well I can tell you from my experince how NOT to heat a bus... First I tried a small portable porpane heater and that thing was a compleate joke. You could only feel a small amount of heat right in front of it, I don't think it raised the temp of the bus much if at all.. you can read about that adventure here viewtopic.php?f=11&t=2472&hilit
Then I got one of those wall mounted ventless propane heaters, which puts out a ton of heat and works great... untill it's time to lay down and go to sleep. All the heat floated up to the upper 1/3 of the bus and left me litterly frozen all night. You can read about that adventure here viewtopic.php?f=11&t=1532&hilit
When that failed to keep me warm camping in Febuary in Canada I ran the engine all night with the bus heaters going... and that works great.
I think the best solution for heat in a bus is an actual furnace. I've seen used house furnaces go for as little as $150 at local auctions and craigslist & RV furnaces for about the same. I think this is what I'll try next since I haven't found a good way to heat my bus yet.