Ok, so let me provide some examples:
Heartland Big Country BC 3850 MB is advertised at 13' 3" exterior height, no mention with or without AC. Flat roof. I have no headroom in that model.
Jayco 377RLBH is advertised at 13' 2.5" exterior height, with AC. Arched roof, even at the edges I have more headroom than the BC and about 3" in the center.
There's a limit to how low the master floor can go, I'd presume, given hitch box compatibility and truck bed rail clearance, so the headroom is in the overall height?
Just confused, since this disproved the theory that full height models correlated to more headroom in master...