Pulling a trailer is always going to hurt your mileage, especially in gas trucks. Thats why the truck guys always ask why you need a diesel... you pulling something? The diesel will do better for pulling something. Those new trucks are rated at 22 mpg because that is what they got driving 55mph down the highway for 300 miles with the cruise control on. Youre never going to get that even with normal highway driving. I bet you could get 20 mpg with nothing in it if you were driving nice. But In the city I bet it gets 16 at the most. Pulling a trailer you're probably getting in the 15 range or maybe even lower. How many miles are on your truck? Have you made any modifications to help it gas mileage wise? i.e: Chip, bigger exhaust, cold air intake?