My mileage varies so much it's rediculous. Between the summer and winter and obviously city and highway driving.
Summer: on the highway I once saw 23MPG but normally get around 20 when loaded down and driving 65-70MPH. 15MPG city stop and go traffic.
Winter: on the highway I get 18MPG and 13.5MPG city. garbage.
But, it's not necessarily the trucks fault (unless you badly need maintenance). So much of it has to do with the time of year and the fuel. Refiners change the ingredients of the fuel, based on the season. As the weather gets colder, different additives are used to increase the oxygen content of the fuel. Lighter, lower boiling point fuels (high volatility) are needed in the winter to reduce hard starts and reduce emissions, especially when the battery may be cranking less than normal. In the summer, it's the opposite; heavier-ends and higher boiling temps are the goals. In short.