I would bring one of your adult kids with you when shopping. We had a 31' Class C with bunks. We bought the motorhome when the kids were 8 and 6 and the bunks worked great for them. By the time they were in their early teens, they both complained the bunks were too small, too narrow. They would hit the window as they slept.
Our bunks were 28" x 74". I expanded the bottom bunk to be a standard twin size and it worked much better. The over cab bunk had to be used for our other teen and the top bunk was turned into storage.
-Michael