Here is your problem. You have 345hp under foot in a truck (you didn't say RC or QC) that weighs 4800-5300 lbs. You most likely have 3.92 rear gears, which is a decent trade off between getting it moving and mileage. Around town you'll be lucky to get 14 mpg (if that). On the highway if you stick to 60 mph and cruise control you might get 20 mpg. Can you keep your foot out of it? I remember when I was 17 I couldn't keep my foot out of anything (and I had small block Dart to play with). Bottom line, if you baby it throttle wise and keep the speed a little low you will get better mileage, but if you were looking for a mileage vehicle, you probably shouldn't have bought a truck.
2003 Ram 1500 QC Hemi Lonestar "Big Red" (Mods list in profile)
2006 Charger R/T "Little Red"
Sarcasm - it beats killing people