My brand new Cougar does not have a duct in the bathroom either. However, when I am spending much time in there with the door closed, I have the rood vent open and the fan running. That brings all the conditioned air into the bathroom though the large gap beneath the door. When not in use, I direct air in through the slightly open door to keep the area cool.
I was thinking like you initially. Perhaps the designers do not want any air from the bathroom being pushed out into the living space. That could introduce some nastiness.
After a few weekends of testing the rig out in really hot weather, I am good. I would prefer a vent, but the fart fan method of cooling works.
Thanks - Chris
2017 Cougar XLite 29BHS
2014 Ram Laramie 3.92 Axle 5.7 Hemi