Note that while the factory measurement for twins is 34" x 78", in some models there is additional space at the foot of the bed. If the mattress is shoved all the way to the rear (or front on a "FB" model) into the curved area, I could have a really long mattress for the really tall folks. In our Classic 30, we could easily add four to six inches more length.
The center nightstand on the Classic twin model would allow for a ten inch thick mattress and still be a little lower than the flat surface of the nightstand. I think a twelve inch thick mattress would be close to ¾" above the nightstand top.
WBCCI Life Member 5123, AIR 70341, 4CU, WD9EMC
TV - 2012 Dodge 2500 4x4 Cummins HO, automatic, Centramatics, Kelderman level ride airbag suspension, bed shell
2014 31' Classic model 30 twin beds, 50 amp service, 900 watt solar system, Centramatics, Dill TPMS, disc brakes, 16" tires & wheels