We have had a twin size in a refurb room, bldg 2. Just a flat one (not the double high), on the floor near the window. If it helps, we have the Target Embark brand single high one that is around $15 (maybe 8 or 9 inches tall). We could not get the angled door all the way open with the mattress down (not a big deal though), his head was in front of the drink fridge. I can't totally remember, but I think the corner edge was a bit under the murphy bed as well, so I don't think a double height would fit. The front wall of the room where the window was is at a slight angle, so the mattress isn't straight, if you know what I mean. It was totally fine for sleeping, but you definitely can't leave it down all day and still move about the room.
I recently bought an inflatable sleeping pad for camping, so if we travel with the same tween boy/girl cousin team again, we'll probably use that. For ease of packing, as well as for the space saving.