Look for coaches that have the most head room throughout as that measurement will dictate how much room you find in the cabover. A lot of coaches only have 6 to 6.5' of head room and that really limits the space available for the cabover area.
Our Nexus Phantom 23P as 7" of headroom throughout the coach which combined with the 101" of exterior width generates over 8" of cabover bunk length. They also installed screened windows on both sides and and vented hatch over the center. We don't often have guests up there but do enjoy using it for reading, watching TV or other "fun" stuff. :B
