First of all, it sounds like you're calculating your fuel economy the wrong way. What the gas gauge says is not important. It is a representation, and is not always accurate.
Here's how it should be done:
1. Go to the gas station and fill up the tank.
2. Note the mileage, or set your trip odometer to 0.
3. The next time you need gas, full up again, and note the mileage since the first fill up.
4. Divide the number of miles you put on the truck between the 2 fill ups by the number of gallons it took to fill the tank the second time.
That is the only way to obtain true Miles per Gallon. Do that, then tell us what you get for your calculated mpg.
Most trucks get between 15 and 20 miles per gallon depending on the truck specs, your driving style, the type and quality of fuel you're using, the terrain you drive on, etc. There are TONS of factors at play. Get your MPG figured out, and give us some more info about your situation and truck and we'll try and help you figure out if something is wrong.