Sportsmobile can build it, or several of the many van conversion shops, which tend to be small businesses today, in regional markets. Sportsmobile will have more experience with the RV part. If you want three rows of seating and bathroom facilities, you will be building on one of the longer vans.
Another variation would be executive travel vans, Airstream used to have some Sprinter-based motorhomes in this class, probably the most expensive on the market. I've been in others used by limousine services, but these have had more than just the bathroom, also having small bars with snack kitchens. These are custom builds for the commercial market.
Neither type B motorhomes nor custom van conversions come cheap. You are starting at $40,000 to $60,000 for the bare high-top extended-length van. Costs from there depend on the quality of furnishings, ranging OEM seats on up to the equivalent of airline Business Class. School bus or transit van seating is cheaper, but I've never seen it put into a conversion van.
Category you are describing is a conversion van, rather than a type B motorhome, since you are not including sleeping and cooking facilities that put the conversion into the "home" category. That doesn't mean a RV builder can't build it, just that it is not what they usually market.