To verify, the bus ceiling, from floor to ceiling is 6' 6".
And how tall are you?
Besides just wanting a raised roof of some kind, I'm trying to understand why the existing roof isn't tall enough.
I get if you're like six four that you'd like some more headspace, but if you're like even six two, it seems like a lot of work and potential issues to consider any kind of a roof / ceiling raise.
So many other things I can think of to spend your time and money on in a conversion if there's already adequate headroom.