I'm brainstorming here, but why couldn't you sort of "hang" a new insulated shell from the roof? Cut roof rafters to fit the contour of the bus roof then bolt wall studs to the rafter ends. Put insulation board between the studs and bus body for a thermal break then insulate between the studs and cover it all with 1/2" plywood and the exterior finish of your choice.
The inside looks very homey.