Airstream added a single bunk above the rear corner bed on the 23D models a couple of years ago as an option. Pretty slick idea for an extra bed I think. I don't think Airstream calls this a "bunkhouse", maybe the seller gave it this misnomer for a little added value.
Doug and Cheryl
2012 FC RB, Michelin 16, ProPride 1400
2016 Ram 1500 Laramie Crew Cab 4X4 Ecodiesel 3.92 axles
The Truth is More Important Than the Facts