We're pretty similar then. My combined city/highway driving at various speeds and unloaded averages about 16 mpg. I do have the 20" wheels, 2wd, and I am unsure of my rear gear ratio. The best mine has ever done on long drives unloaded is 19 mpg. I am also due for spark plugs, but I know that want help a bunch.
I have had mine since 2010 when I bought it used with about 13,000 miles on it, and fuel mileage is staying pretty darn consistent all this time. I am up to 298,000 miles now.
I would say nothing is wrong with yours since it compares so closely to mine. Do you tow in overdrive or with overdrive off? With a big trailer I turn overdrive off and try to limit my speed to around 65 mph, and less in windy or poor weather conditions.