Basements, not the kind you find under a building but the ones you find in the bellies of an RV. I don't think you can ever have enough basement space. In our KZ Escalade, we have one large basement area that can be accessed from either side of our rig. It has a heavy duty rolling tray which works well...well, unless you get something stuck under the tracks.
We do have other storage areas but they contain the things we need to travel - batteries (another B), generator (where we also store our portable DirecTV dish and a couple of folding chairs, and the LP tanks).
In my older Class C Tioga, I had a lot of basement storage although only one went across the width of the rig and it was so small and narrow I used it for my outside broom, folding rake and the like.
Can you ever have enough basement storage? How do you use yours?
BONUS: B is for Badlands