Express 3500 12 passenger van ? Towing experience?

One data point:
Empty local driving - 13-14 mpg
Empty highway - 17- 19 mpg @65mph
Towing - 11-12 mpg

One odd thing I found when I went from a 5,200lb camper to a 3700 lb camper is
that my fuel consumption stayed EXACTLY; My conclusion is that trailer frontal
area has the biggest impact on fuel economy;

I would expect your fuel consumption towing would be closer to 10MPG as you
are towing approx 1,000 lbs than my heaviest camper.