I used Foamular from HD. Used it in ceiling and skinned windows (2-- 3/4" layers for 1.5") and furred out for 3/4" on all the interior walls which gave 2.25" behind all the skinned windows total. Figured I could give up another 1.5" for the comfort and sound control. Worried about squeaking, but the foam is bonded with Great Stuff and adhesive which seems to have worked well...no squeaks. FWIW, some foam boards are not compatible with adhesives such as Liquid Nails.
Because I'm a cheapskate, I put all the aluminum ceiling panels back up once insulated...worked out fine, but made installing a headliner more complex...so complex it ain't happened yet. It will.
As Kristinsgrandpa and Jack sez, by the time the foam is burning, the toxic fumes will be the least of my worries. I'll be out of the bus...or a crispy critter.
The more insulation the better! No doubt about that. Worth the effort and expense.
Now the only area that has heat load or loss is under the dash. That doesn't have much effect on the comfort of the bus. Have heavy curtains on tracks that pull over the windshields that are a wonderful thing when camping in really hot weather and the bus is facing the South.
BTW, Foamular advertised an insulating factor of R-8 per inch...been some discussion on exactly what conditions the R-8 would perform?