Our 2015 ERA 70C has a Fantastic Fan in the bathroom (dry bath, real toilet, real shower) and the standard roof top AC unit, no ducts.
A jigsaw and few tubes of Dicor you could maybe you could add a fan?
My guess is that fitting an exhaust fan is that is IN the bathroom on most class B have a wet bath located on one side or the other and the roof is not flat enough to mount the fan that close to the edge of the roof.
History does not long entrust the care of freedom to the weak or the timid.
~General Dwight D. Eisenhower