Think along the lines of a house.
Air infiltration...wind...will take away a LOT of heat. Seal holes with good caulking not just generic crappy silicone.
Get rid of as much glass as you can. If not, insulate the windows.
On a tear drop trailer forum someone suggested this material:
Superfoil SF60 multi-layer the ultimate multi-foil for roofs + walls.
I think that you'll find the foil clad foam sheathing is the best insulation, inch for inch. If inner dimensions/height is critical that is.
If looks don't matter, start the build and seal it with plastic wrap or blue tarps. Not very durable or good looking though ;)
If you're not going to move the bus adding some skirting will keep the wind from blowing up your pants leg. Maybe straw bales...but I've been suspicious of them being mouse attractors.