all this debate on painting the roof... and it's only going to serve one purpose
personally, I'd lean toward putting up an almuminuminumnumum framed deck floating 2 inches above the highest part of the roof, insulate it with layers of 1/2" plywood*, 1" double foiled insulated foam board, refletix, with a 3/4" layer of plywood on top, with aluminum cross bracing, weld stake pockets on the main aluminum frame, with stakes for a removable canopy with sun shade fabric... or make a screened porch for catching the breezes, when the weather is nice.
and turn that deck into dual purpose, like the upper deck of houseboats, with airflow under the deck.
Start the aluminum frame just behind the driver's cabin, and put solar panels above the driver's cabin, on tilting frames you can tilt them from the upper deck. Lay flat during transport, and tilt when you're on location.
a short 3' - 4' rear deck mounted to back of bus for a staircase /or\ ladder on one side, and opposite side of deck for the lower part of a mini-split A/C unit with dual zones for the back half and front half of bus.
paint it all you want, but you're not going to be walking around on top of it much!
*you could even use expanded steel for the underside of deck, if you prepare it for electrolysis because of differing metals.
as soon as I posted this comment... the images that refresh at the top of the page showed this?