We have lots of unused storage space. Mainly that's due to realizing we don't need everything. But, we are the type that bring carryons only for week vacations via jet.
My mattress is a very thick heavy queen. I'm not sure how accessible under bed storage would be. I liked the under bed storage on the class A we had. I can't say I needed all that we carried. On our camper, the walk around space is closets and side windows. As others have noted, it would be harder to get in and you lose room to sit up.