B+ usually has cabinets because thats's all the space there is up there. Most I've seen would be hard pressed to fit a sort, cot-width sleeping area in that space. If you want bunks up there, look for a C with bunk-size section over the cab.
If all you want is storage, you can get as much space as the OEM cabinets used, which for some models is a lot, others not so much.
Tom Test
Itasca Spirit 29B