Slow down, coast A LOT more often, slipstream big vehicles when you can and don't lug things around you can leave at home. You should look up hypermiling, it is a technique commonly used to get better MPG then posted dealership figures. After hypermiling a V6 GTI that should be getting 25mpg according to volkswagen. I got 38.
My experience with the large Chrysler V8s (4.7 and 5.9L) is that you mileage is directly related to how much stop and go you driving you have. These guys suck gass when accelerating. If you're getting that MPG on the Hwy something isn't right, but if you're in town 10 to 12 sounds about right. In one spread out town of 50000 where there is no real downtown and the stops are spread out every 5 or 10 city blocks I can get 13-14 MPG. In a town of 200000 where the stops are every block I get 10 MPG. On the highway I get 20 MPG at 55 MPH and about 17 at 70 MPH. This is with a 4.7L 5 speed. The 5.9L can do better on the highway with the cylinder deactivation if you keep your speed down closer to 55 MPH than 70. If you're towing much of anything then your mileage is probably about right as well. BTW both of these motors prefer extra octane. They run better on 89 and will yield slightly higher mileage on it (especially when towing) but the gain isn't enough to justify the added cost.
Thanks for all of your responses. I haven't towed anything and don't carry much. Usually it's just me in the vehicle. It is a V8 AWD so I knew the fuel efficiency wouldn't be good but I needed something to tow in the summer.
MMUK - I live in West Michigan and it's been in the 20's and low 30's lately.
MatrixDom - It has been running while parked a fair amount. I didn't think it would make that much difference but I will stop that and see how much things improve.
wr - My driving is almost all in the city with start and stops all the time.
I don't drive very far to work so my mileage isn't going to hurt my finances that much but I still want to be as efficient as possible. It sounds like I am getting close to what I should be getting. I will look up some hypermiling techniques and see if they help.