boat gassed when towing?

I've always towed "long distance" with and empty boat gas tank. The lake house is 120 miles away across the flat Texas north east. This time, it would be a major convenience to fill the boat up before we leave. I drive an '06 F250 diesel. The boat is a VLX on a tandem trailer. With this truck, do yall really think that my gas mileage will be affected by the full boat gas tank?
I can never remember if it's affected or effected.....