I know it can be done; but has it been done? It would be nice for someone wanting a toy hauler; but not a large Class C. Just enough room for a motorcycle or 4 wheeler. Or even having that ramp doo...
I haven't seen a B+, but there's a couple Cs out there, and you could just turn the bunk into cabinets. For new, there is the Thor Outlaw:
Prior to this, Thor made the Funmover in 27 and 31 ft flavors, neither with a slide out. They did have some neat options like a full liftgate, instead of a tailgate.
Also, Bigfoot used to make a "garage" model called the 29G, with a large under-bed storage bay large enough for a quad: