Generally, expect to see figures in the range of 14-18 mpg city and 17-21 mpg highway. However, this is just a rough estimate. Actual mileage will depend on driving habits, vehicle condition, and tire inflation.
To get a more precise idea, you should look up the EPA fuel economy estimates for the *specific* engine and drivetrain configuration of the van in question (e.g., 4.3L V6, 2WD). You can often find this information online through sites like fueleconomy.gov (though data may be limited for older vehicles) or in the owner's manual.