OK, I'm seeing it, now. You're looking to scalp your bus, then frame in walls and a roof using lumber. My bad. I thought you are planning on a typical roof-raise using atypical materials in the resulting gap.
I reappraise my initial reaction. While not as dire as I first imagined, I don't have warm and squishies about it, either.
Part of the appeal of these massively clunky machines is their inherent safety. Over-engineered to perform their intended daily function, it's the, "What If," factor of an accident designed in that increases the otherwise aesthetic and/or funky appeal of a bus conversion.
An extra 2' above the deck is at the high end of conservative, but not unheard of in the more traditional roof-raise. 12"-18" is much more commonplace.
No, my concerns relate to the resultant structural integrity, not building material's weight. By removing the transverse ribs, or rafters, the crush factor in the event of a side impact, be it by roll-over or t-boning, is significantly increased.
Theoretically, what you propose can be done. I've never seen the like done to a bus in person, tho I have seen cabins framed onto pick up and van bodies before. Very solidly constructed, structures that could survive and shrug off the worst of the stresses from feet of snow covering that the north woods could throw at them.
I've also seen the piles of kindling & matchsticks strewn about following a wreck...
It's been a long time since I worked framing and finish, but I'm thinking regular gypsum drywall'd be a bad call, given the condensation problems Skoolies are prone to. Seems there was a product (green board?) that was used in bath/shower encloses. Water repelling or resisting drywall'd be the way I'd go, if I were going your way.
But I'm not...