Well, as a former fiberglasser (maybe not completely "former") I prefer epoxy. The "real" stuff, not the "5 minute" cheap disposable packs you get, which are the Elmer's white glue of that family of adhesives. Any boat or most major home improvement stores carry small cans of "West" 207 or similar. That will last you forever. You can even make structural attachments, or make body-panel repairs, with the right additives. If you lightly sand both surfaces first to provide good adhesion, those things are never coming off...
My next favorite is "Gorilla" polyurethane glue. This stuff expands so you get squeeze-out if you aren't careful, but it's incredibly strong and will hold up to heat very well. Like epoxy, it's also waterproof, which could be an advantage on the inner surface of your bus (although if you're trapping moisture in there you're in trouble anyway...)
Both of these can pretty much only be removed by sanding if you change your mind later (or solvents you don't want inside your bus...)
After that, I suppose Liquid Nails has a few options. I tend to think of those more for large surfaces like wall-board and flooring, where being able to shoot it out of a caulk gun is a big advantage. Also, some of these varieties can be scraped off if you change your mind later...