How about a Chevrolet Kodiak based Super C with 8.1 gas and Allison transmission? We had a 2008 bunk bed layout. could easily tow a mini-van (or possibly something larger), Had 10 seat belted postions, could sleep 10 (2 adults 8 smaller humans).
This STILL may not be enough for you if you are a family of 13 though..thats a LOT of people in a single RV.
I have seen some older "tour RV's' that were set up for travelling bands or something similar...multiple bunk beds, with small living space.
Good luck, your looking for quite a niche vehicle.