Driving 70/80 miles an hour really tends to burn up the gas.
That's the main thing that's bringing you down on the mileage.

Couple of other things I noticed.

Off road type tires tend to have more rolling resistance, decreasing gas mileage.

Caps don't help gas mileage. In some cases they can make it worse by creating a very large dead area behind the vehicle. The gap you describe between the cap and the cab aren't going to hurt you.

Lifting a vehicle may adversely affect air flow under the truck. This can cause gas mileage to go down.

Are your front hubs disengaged?