There's really no magic bullet (other than the magic bullet of a roof raise) - you just have to trade headroom for insulation value in whatever quantities work for you. My ceiling is 6'7" down the center and I'm 6'1" in shoes, so I'm putting 2" of XPS foam on the floor and 3/4" plywood; on the ceiling I'm putting 1.5" XPS foam between the ribs and then 3/4" XPS over everything and 1/4" plywood. So I'll lose 3" off the floor and 1" off the ceiling, leaving the bus 6'3" down the center, just enough to keep from hitting my head.
My kitchen has a dropped floor 3" below the rest of the floor (and the bottom of my shower will be another 6" below that) but this was only really practical because I had to rebuild that section of floor due to rust anyway. I don't think it's something you'd want to do to a healthy bus floor, but some people have.
A number have folks have done either a dropped shower pan or a little raised box on the roof above the shower to give some extra headroom in the shower stall, at least. I guess I could see making do with a living space that was slightly too short for me, but having to shower while bending over would completely suck.
