That's awfully low if you're talking about a diesel. My '11 gets 21 freeway, 15 city, and 16 towing a 6k lb boat on the highway. Got around 14 towing 10k once according to the lie-o-meter. I can't imagine what I'd have to tow to get 8mpg with the new diesel. Sounds like the mileage a POS Dodge would get while billowing black clouds of smoke and sounding like a whoopy cushion.
Edit: You may want to have your dealer verify you have the latest flashes that increase horsepower and smooth the 4-5 tranny shift as well as changing shift points on the others. My mileage increased after the TCM update.