I used the Foam Factory/Foam by Mail:
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to order a custom mattress to fit the bunk. The one I ordered (2" memory foam and 3" standard foam) is far too thick (5" total) to fit on the bunk when its fully up. Our solution is to just throw the mattress on the rear beds and when we're parked put it on the front bunk (only raising the bunk high enough to get it out of the way during the day). Granted it isn't a perfect solution but it is more comfortable than the air mattress.
The front bunk really only has about 1" - 2" of lip protruding up from the bunk thus anything you'd get that would fit there would probably not be very comfortable.