Thomas body buses have a tall roof model, but they are hard to find in the short bus style. I have a 30 foot long tall ceiling Thomas bus. The ceiling is 6'6" from bare metal floor to ceiling. Once you start adding flooring of course you start losing head room. I put down 3/4" of flooring and I'm 6'4" tall, so I have just an inch or so of headroom left.
I have a 1993 Genesis 25' flat nose and it's 6'2" from the bare metal floor to the middle of the ceiling... Mine seems to have less head room than a lot of others... Everyone in our family is under 6' tall so we're good...
I also have a 25' flat-nose Genesis. But mine is 6'6" tall inside. But it's a 1995 model, so go figure! I think you will have to just find one you love and make it work for you. You could always just raise the roof. That way you would know for sure that it is tall enough.
I went to considerable lengths (heights?) to find a high (6'6") ceiling bus that also met my other criteria. But then, I'm 6'4" and have back problems, so I have no intention of hunching over while hanging out in my bus. I recently stepped into a bus that had a 6' ceiling and it was intolerable for me to be in there for any period of time. If I were full-timing I think I'd have to raise the roof. Even in my bus, with the flooring I added, my head is less than an inch from the ceiling.