Spray foam can stick pretty well -- they use it on the outside of the space shuttle fuel tank. Oh... Never mind.
Seriously, it might work nicely under a bus, and spray bedliner material is good stuff too. But I agree with Demonknight about getting any kind of "undercoating" on the hardware that you may -- make that WILL -- need to work on later. It's hard enough working under the bus as it is.
So I would raise the roof, yes. Oh... I did. See The Millicent Chronicles under Projects.
Then insulate floor and ceiling to your heart's content.