I have a class C and I love my cabover bed. It can be a pain to get in and out of if you have mobility issues. For young adults, it would work fine. For the 9 year old, it would be awesome.
Are you staying at campgrounds or RV parks? If at campgrounds, I would toss in a tent for those times the older kids want to come. Better yet, buy them their own camping gear for holidays/birthdays. They can camp with friends and join you as they desire.
Get a model with the cabover bed and a queen bed at the back. Put the 9 year old in the cabover and you and spouse in the queen bed.
If you are staying at RV Parks, then you're going to have to make concessions as many don't allow tents. Using a dinette convert for the older kids if they come, or putting them in the cabover and putting the 9 year old on the sofa.
There are bunk bed models, but from what I've seen they tend to be on the smaller size and a bit claustrophobic. I'm not sure you can find a bunk and queen bed model.
