Forum search engine didn't return any results for roof vents on Das Bus. Are there any? A powered roof fan and vented windows would help quite a bit.
There are also thermal insulting paint / paint additives that can be used on the roof.
My brother says the solar on his family's live in school bus is enough to run an air conditioner through the worst parts of the day, but they are still relocating to cooler elevations after spending a couple weeks in Nashville, TN last month, about to visit Black Hills attractions.