Advice needed on F350 V10

I finally found the V10 I was looking for. 2001 F350 Lariat dually, crew cab, 4x4. All the bells ansd whistles. Excellent shape. Bilstein gas shocks, Firestone airbags with onboard compressor, oversize tires, the works. I talked to several people who said gas milage would be around 14 mpg. I've never been a gearhead so didn't know much about the larger rear ends. The owner thought it had 411, the book says 430. All I know is I was a little shocked when the gauge showed 6.9 mpg. ?? Besides a K & N cold air intake, which will be my first purchase, what can I do for better mpg? Had I known about the milage I probably wouldn't have bought it. My fault for lousy research. Sorry for the long winded intro. Thanks.