I just made a trip up to NJ up 95 and averaged 20 and thats at 75 mph with my adrenaline set at 3Bigger exhaust.
BHAF air filter.
TWC3 2cycle oil in fuel at 1oz per gal.
Keep highway speed at 60mph or less.
Easy accelleration no full throttle starts.
If you have a pyro gauge try to keep your exh temp below 650.
Keep it in the highest gear without lugging the engine. (Drive like an old lady)
Don't let sit and idle when you don't have to.
You will get better milage when it gets warmer out and the fuel is a summer blend.
Your driving habits have more to do with milage than truck modifications do in most cases.