Something definitely needs attention if your mileage is that low. Either the engine is running rich and burning too much gas, or the rolling resistance is high, or there is a problem in the drive train. Tune up of plugs, air filter, clean the throttle body, MAF sensor, and the injectors would probably keep the engine running properly. As for rolling resistance, tire pressure, check the brakes to make sure the pads are not dragging, check the parking brake to make sure the shoes are not dragging ( the lining on the shoe can separate from the shoe ), change the differential fluids and transfer case fluid. Check the wheel bearing, sometimes a bearing getting ready to fail gets harder to turn but doesn't grind or feel lose yet. If all of the doesn't improve things, check the transmission ( or have it checked ) to be sure it working properly.
I recently picked up a 2002 Mercury Mountaineer with the 4.6 V8. It was in need of a lot of work, had misfires, coolant leak, bad wheel bearings and maybe got 10mpg. After new plugs, a couple of COPS, cleaned the injectors, throttle body, MAF, air filter, Fuel filter, new intake manifold, 3 new wheel bearings, new tires, new brakes, changed all fluids, and so on, it is now getting 22MPG while on cruise control at 65mph.
Sounds like something needs service on your vehicle, good luck tracking down what the problem is.
