A typical bus has ribs that are 1.5" thick (the space between the ceiling and roof panels), so 1.5" XPS foam fits well although it doesn't bend enough on its own to handle the curviest parts of the roof without kerfing or piecing. The ribs are typically about 27" on center so the gaps between are about 25.5", a bit inconvenient for 4'x8' sheets.
Rockwool is available at the big box stores typically in 1.5" thickness and it's bendy enough to handle the ceiling, but it's usually in 16" wide batts which doesn't work great with the rib spacing. Slightly lower R-value than XPS, but it's better acoustic insulation.
Well-applied spray foam gives you the insulating value of foam and also seals the cavity from moisture, but it's pretty expensive. Seems most people need at least two of the kits which is about $1300. Some people who DIY this have stated that they wished they had hired pros to do it - the good news is pros often cost about the same as the kits. You need warm temperatures to apply it, and you have to worry about "oil-canning" when the pressure of the expanding foam bulges out your side panels.
A general impression I get is that about equal numbers of people on skoolie.net do XPS foam board (or less often EPS or polyiso) and spray foam; rock wool is very rare. Stuff like denim insulation is even rarer.