The Dodge/Cummins combo gives outstanding fuel mileage but no Dodge diesel is getting 20 mpg AND pulling heavy loads. Maybe they are pulling 3-4000 and getting 20, but certainly no more than that. I would guess they get about 13-15 with your size of load.
On the other hand, 10 mpg sounds a little light for your configuration. I pulled 11,000 lbs with a 99 F250 7.3 Powerstroke and usually got in the 11-13 mpg range. But maybe you have a heavy foot. It does't take much to drive the numbers down.
Another thing: what is the total weight of the van + trailer when loaded for the road? You can put a lot of gear in a van!