True bunkhouse (4 teenager sized bunks)

Not sure what a teenager sized bunk is? My 18 year old is 6'1" 180lb and he fits fine in the 'small' 28" x 75" bunk in my TT, head height is a bigger deal than width for him so he gets the top bunk. From that perspective you're going to probably need to get everyone to actually go and sit in the bunks to make sure everyone has enough headroom.

As an aside, standard navy rack is 27.5 x 77.5 (mattress is 26 x 76) and plenty of full sized adults have served. Personally I'm 6'3" and 250lb and I fit into a navy rack fine (air mattress is nice though!).