We were just talking about this exact thing today. People build huge expensive, overly complicated buses with slide outs for their bunks. All to have larger living quarters and still have real beds. I say the real answer is to have a huge lounge, kitchen, bath, and master bedroom in the main coach where you spend most of your time. Then you can trailer your bunk house behind the bus . Every one can have a bigger bed, and it will be way cheaper and faster to build.
I have seen this exact thing done on another forum where the truck had a rear door that lead to the hitch. 1 step across the hitch and you where in the door to the bunk room. It even had it's own bathroom! Also if it was a day trip the bunk house just stays home! People need to think outside the box...or the bus as it may be ;)