You'll find a range from roughly 10 MPG to 18 MPG in real-world driving conditions. The lower end of that range is more likely for heavier configurations and less fuel-efficient engines (like gasoline V8s). Higher MPG might be achievable with a lighter van and a more fuel-efficient engine (if such an option existed for that year, which is less likely).
To find a more accurate estimate, you need to know the specific engine and configuration of the 2003 Ford E350 you're interested in. The information should be on the vehicle's sticker or in the owner's manual. You can also consult the EPA's fuel economy website (though data for older vehicles might be limited).
