No basement in Class B's or camping vans. Largest vans are 24 feet long, less than 80 inches wide, probably a half foot less interior after insulation and trim. Queen size beds are easy in that space, take up less than 1/3 of the area available. But if you are coming down from 200-400 sq ft of space in a Class A, to 120-140 sq ft inside a 24-foot van, all spaces become dual purpose, convertible, except that doesn't work for cooking and bathroom facilities, which detract from the space.
Look at the living space options in the Winnebago ERA models, and some of the Airstream cruisers, for the best interpretations of using this small space for a variety of different needs (party van to full-time living). Downsizing this much in a RV is about downsizing your lifestyle. A 400 sq ft lifestyle does not usually fit into a 120 sq ft living space.
Rather than a B van conversion, you might find better alternatives in small Class C (or B+) or even smaller A's built on the 24-25 foot Sprinter chassis, with 7 to 8 foot house width. That gets you closer to 200 sq ft living space, rather than the 120-140 inside a van body.