I've recommended haunting RV and regular junkyards and pulling the furnaces out for use in skoolies. I've paid $20, others have paid $50 for furnaces...depends on if they know what they're selling or not.
Just make sure the burner chamber isn't burnt through......
The furnaces run off propane, and many have blowers for forced hot air; they also use thermostats, get an electronic one. The blowers can/may run off of 12VDC or 120VDC, depending on how it's equipped.
Keep the propane tank OUTSIDE the bus! If running only the furnace, I'd use a regulator salvaged from a gas grill...cheap, yes?
Connect the propane with flexible copper tube of appropriate size, and use flare fittings, NOT sweat copper joints.
Another nice thing is you can heat the ends of the bus more readily by using standard 3- or 4-inch insulated flexible ducting, available at Home Depot or Lowes.
Otherwise, wood stoves are sometimes used, but they have bulky wood as a drawback, along with the chance of CO poisoning and setting the bus on fire
Some people love ventless heaters, I fear them for CO potential. Take your pick.
BTW, other junkyard goodies include rooftop AC units, windows, water tanks, awnings, stoves, water heaters etc.
DO NOT USE LOOSE HEATERS IF TRAVELING/MOVING. IT MAY FALL DOWN AND SET SOMETHING ON FIRE!!!!!!
Good luck and happy motoring!