twowheels, I'm assuming this is for a child, right? The weight limit on those bunks is about 140 pounds, if I recall correctly. We took our bunk out when we got the trailer and replaced it with a cargo net for bedding.
Again if I recall correctly, the bunk platform folds up for storage. I am pretty sure that our mattress folded up, too -- otherwise, how would it fit after the platform is folded??
The mattress was very thin -- maybe two inches thick. If it is too thick, then it would be difficult to fit the folded mattress on top of the folded bed platform.
We did not use the bunk as a bunk, but I did wonder how the occupant of the bunk would get down from the bunk in the middle of the night to use the bathroom -- the kid would have to step down onto the main bed, and hopefully not on the parents' legs!!