I used two 3/4" Foamular panels for a total of 1.5" in all of the ceiling and skinned window areas. Foamular came from Lowes. Also used in sidewalls. I don't know how the Foamular compares with the other panels mentioned, but the bus doesn't squeek, and it cools readily. One AC will keep the front cool in any sort of weather. Bedroom will get a little warm if rear AC is off when the sun is on the coach and it's 100* outside, but once the sun sets, the rear even stays quite cool on just the front AC. The bedroom has its own AC anyway.
Found the 3/4" panels easier to form around the roof curves. The 1.5" panels would work in flat room areas.
This stuff is a good bit of work to install nicely, but it won't buck the sides of your bus. And, there's no major "shaving" or cleanup once its in.
A word of caution...if you use an adhesive to hold the panels, be sure that it is compatable with your foam panel material. I used both "Great Stuff" (Great Stuff WILL bow any panel... and fiberglass shower stalls
) and sealer to seal around some of the edges so the foam wouldn't rub and squeek when driving.
I believe the panels ended up burning thru about $600 bucks...it was a noticeable amount. Worth it for the quietness, and comfort.