In our A122, the thinness of the stock mattress also affects the fit of the sheets - and made the bed quite uncomfortable for a night's sleep. We added foam toppers to both the bed and dinette, and now they both sleep wonderfully. And the extra thickness makes the sheets fit better.
I would measure the actual bed size, and get as close as you can. The A-frame beds that stretch across the width of the camper are generally 80" long, so an extended full or extended twin sheet is necessary if the bed isn't the standard queen 60" wide.
just our experiences
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan