Curiosity got the better of me. 22 gallons is what I found for a '79 Econoline. And roughly 432 miles for your trip, Jon, which came up to 19.6 mpg's, but that was if you used the entire tank, and if the map gave me the correct beginning and ending points. North of 15 mpg in a loaded van is still excellent for a rolling brick. 24 mpg if you had two gallons left.
Okay, I will stop posting now, I got carried away, my apologies for the parts that were off topic.
