I haven't applied spray foam to a bus, but I have been around other spray installations. I think that it winds up very lumpy, and you have to lose a lot of precious space to make the paneling work. If you go with the sheets, you can "laminate" very thin plywood on to it with construction adhesive. Maybe use Great Stuff, or equivalent, to foam around the edges of the panels to make them airtight.
I'm thinking about doing a similar lamination thing with Homasote sound board, to cut down on noise and insulate. Gluing on cloth to 1/2" Homasote could make for a nice headliner- the company sells panels covered with burlap.
In my house, I built an interior door and built-in bookcase rear panel using 7/32" Sandeply (from Home Depot) and 1/2" Homasote. Really stops the sound transmission, and feels very solid. Though Homasote doesn't seem to have much in the way of R value, it's 1/5 that of rigid foam insulation, and more water absorptive.
Maybe painting the roof white would help?