Closest available today is the Express 3500. Chevrolet trailering guide shows GCWR 16,000 pounds with 6.0 Vortec, 17,000 with the 6.6 Duramax.
3500 12-passenger van is about 6300 pounds empty, extended 15 passenger van 6,700. The 12-passenger will tow 10,000 empty.
Problem is, you are going to load the thing up with passengers and gear, and when you get it up to 9000-9500 pounds, you are going to have at most 8000 left for your tow, and that's with the diesel. You would need to find a 6000 pound trailer that fits your needs.
Next option up is having a transit-type vehicle built on a Class 4 or Class 5 cab-chassis, or finding one used. Ford is still selling the E-450 to up fitters, V-10 gives it GCWR of 22,000. If you can keep your loaded transit vehicle under 12,000 pounds you have 10,000 for towing. However, this sort of vehicle can be problematic (because of parking) as a daily driver, and the models with RV construction are a significant passenger safety compromise. If you want to check out what can be built, Thor, Winnebago, and Forest River all have divisions that make these in various styles ranging from plain bench seating to party platforms. There are also a number of smaller companies in this business.
Shorty school bus or used transit bus? That's trickier, those built on van cab chassis usually have a lower GCWR than RVs, because towing is not an expected use for the buyer. Typical builds are on the lower GCWR E-350, G-3500, Sprinter 3500, and more recently the T-350 which doesn't pull any more as a cab chassis than it will as a van. You need to find E-450 or G-4500 chassis, or something even larger like a Freightliner M or Business Class.
Look at nwbus.com, tescobus.com for an idea of what is on the market. Look at stock number F27419 at Tesco, a Family Coach built on E-450.
