It's hard to calculate gas mileage with any accuracy if you only burn half a tank.
But if you get 23 mpg going 75 mph, then you could potentially get 25-26mpg if you slowed down to about 55 mph.

 

Others have covered this. In summary:

• Check tire pressure
• Check and replace air filter
• Use lowest viscosity oil specified for your truck
• Service transmission every 30k miles
• Accelerate slowly and smoothly
• Use 87 octane (unless you are chipped)
• Don't idle for more than 2 minutes
• Slow down
• Change fuel filter per mnfr's recommendation
• Replace power steering fluid
• Put truck into neutral at long stops

In general, maintain your truck and don’t forget the often overlooked items like power steering fluid, fuel filters, and transmission service. As fluids age, they oxidize and usually increase viscosity producing increased hydraulic drag.

 

I try to drive downhill as much as possible. Best thing i've found is to park it and drive my wife's taurus.