2003 Dodge Ram Bad Gas Mileage

I own a 2003 Dodge Ram 4WD Quad Cab 5.7 Litre Hemi(104,600 KMs)and am currently experiencing HORRIBLE gas mileage even for a 5.7 Hemi.I've done extensive research on the gas mileage for this particular vehicle and have discovered that I should be getting in the area of 12-15 mpg in the city and around 17-20 on the highway. Right now I'm currently getting 6-7 mpg which is obviously horrible even for a Hemi. Air filter is brand new, tires are all properly inflated, and I regularly use fuel injection cleaner.I contacted the dealer from which I bought the truck and they suggested that it could be the spark plugs which might be in need of replacing.I bought the truck with 99,350 KMs on it so I'm not sure if the plugs were ever changed by the previous owner. Suggestions?