Diesel costs considerably more than gas (at least around here), so it would have to get much better mileage to even break even, and then where is the advantage?
For a full sized van, it gets about the best mileage, 22-25mpg might not seem like much on the highway. But compared to a loaded gas van with a V8, it is decent.
There is a reason those Fedex, UPS, and delivery vans are using the diesel Sprinter van.
And if you've seen those vehicles, the passenger van will definitely carry 4 people and all the luggage for a long trip. Unless you want to go to a full-sized SUV and eat it in the wallet for the gas.