I have a 1989 six window Amtran "Ward" short bus. It is dog-nose built on a chevy truck frame, not a van-type chassis. No wheelchair lift. I measured 15 feet of buildable space from behind the driver's seat. Ceiling height is aproximately 62-1/2" from clean metal floor (after rubber flooring removed. After I added 1/2" of styrofoam insulation, 1/2" of plywood and 1/4" of hardwood laminate floor, at six foot tall I still move comfortably around the bus and never give ceiling height a second thought.interior width is aproximately 86" after bead-board walls.
In that space I have built two 80" long couches facing each other that convert into a king size bed, an upright fridge (about 24" wide) and a bath with full size composting toilet and shower. I have overhead storage and will build the kitchen counter/cabinetry shortly to go across from the bath. It will be enough space for my wife and I to travel comfortably with occasional guests for dinner.
I have included a link to some pictures on google drive. They are not in any real order and I have not uploaded pictures in a while but maybe it will help to see the space I am working in. I've been working on this off and on for about a year and a half.
Hope this helps!