In our 2014 31' Classic twin bed, the 8" Rocky Mountain mattress is perhaps two inches lower than the center night stand. The ten inch would be about level with the top of the night stand.
We have the 12" King Rocky Mountain mattress at home and like it very much. We have the ten inch in the 23D and would consider the ten inch twins for the Classic in the future. We sleep well on the 8" twins, but there is a perceptible change in comfort going to the ten inch.
With the twins, one can take the factory mattress drawing and actually measure the existing platform in the trailer. It would easy to add four inches to shove the head of the bed to the rear wall (on the Classic) and have the foot of the mattress even with the bottom of the board at the front. For taller folks, another six to 8 inches of length could be added before touching the wall. If one orders the longer mattresses, remember to change the sheet sizes as well.
It is still an arm wrestling match to make up either the twin beds in the Classic or the 23D corner bed.
I forgot to mention that the original factory mattress in the 2013 25FB International Serenity was a 7" mattress. We had ordered the 600 thread Airstream cotton sheets which actually fitted the 8" Rocky Mountain mattress perfectly. We also ordered the same 600 thread Airstream cotton sheets for the 8" Classic twins and they fit properly. These are really great comfy sheets in cooler weather. We ordered 600,mthread cotton custom sheets from Rocky Mountain since we ordered a ten inch mattress and they work, but are not the same fitment or comfort.
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 w/ twin beds, 50 amp service, 1000 watt solar system, Centramatics, Tuson TPMS, 12" disc brakes, 16" tires & wheels