To get an extra storage area there is a fairly easy to get the room. Raise the bedroom. With the usual bedroom you can almost stand on the bed and still not hit the ceiling. Unless you plan on standing on the bed regularly just raise the level of the bedroom floor and that extra vertical space can be used in the storage area, basement, below.

Some regular motorhomes uses this kind of idea. Bigfoot, Tioga or Jamboree 26Q and the Gulfstream Endura 631 model use some variation of this idea.

Also you might talk to Transport Designs, they have a web site. They had a picture of a truck conversion they build that had a raised bedroom with a large storage area below it. They build custom rigs so they could build anything you wanted.