The newly updated Ford Transit Wagon with the V6 ecoboost is rated 14 city/19 hwy/16 combined... not too shabby compared with the outgoing Econoline E150 which got 13 city/16 hwy/14 combined.
The Chevy passenger vans get 13 city/17 hwy (combined unpublished, but I'm guessing it's probably 14-15 mpg.
Obviously with these low numbers, a little change goes a long way towards fuel savings.
If I were going vans right now, I would want a Transit Connect. The long-wheelbase version is rated 1mpg better than the previous MPG winner, the Honda Odyssey. Seats 7 though, so not for you.
If you really need 12 passengers for you family, your best bet for saving fuel in the future is maybe to stop having children
.
Sam