So the decks on buses seem nice, although with a roof/shade do you really want to be up there, and then they take up solar panel space. So a popup would solve both, put the solar panels on the popup, then you get shade for your upper deck. Don't know about making it part of the inside of the bus, all the heat would go there, not where I'd want to sleep, and in the winter, no insulation from the tent structure. Maybe in Hawaii, it would be perfect.
I'd also like to point out that stuff on your roof really effects your gas mileage, although apparantly nobody really cares about that? Or the ecological effects of propelling 16,000 lbs around the country spewing diesel particulates. :ducks