Yes winter gas will cause a drop in MPG, but there are other factors as well.
Colder weather requires a richer mixure until the vehicle warms up and the colder it is, the longer it takes to get to operating temperature, so you burn more gas in colder weather.
Traffic is another factor. The holiday season brings out the shoppers and the traffic everywhere increases. That means more stop-n-go traffic, catching more red lights, and more slowing down and speeding up. All of them are MPG killers, especially in a 4300+ lbs truck. It only takes catching one or two more traffic lights on your daily commute to impact your MPG.
Low tire pressures are more common in the winter time when the temperature drops and the air in the tires contract...Just a few pounds of air pressure can impact your MPG.
BTW, no additive is going to improve your MPG, and no additive is going to take any additional ethanol out of your gas. More Ethanol is added to winter gas because it mixes with water and prevents any moisture or condensation in your fuel lines from freezing.
...Rich