Horrible Gas mileage

With all the discussion of getting bigger tanks bolted on, I began wondering about the law of diminishing returns. A gallon of gas weighs 6 pounds. Adding another 30 gallons is 180 pounds of additional weight that has to be transported. This contributes to the rolling resistance (mainly from tires). Presumably, air resistance remains unchanged. Rolling resistance is the dominant contributor to fuel economy at freeway speeds. At a rolling friction coefficient of 0.7 (dry road), a 5 ton RV (10,000 pounds) requires a certain amount of force to move the RV along—independent of velocity. We all remember Newton's second law of physics: F=mA. Factoring in rolling resistance to air resistance without the extra weight would be F=10,000(A), with the extra weight F=10,180(A), Hence, the difference appears negligible. Those who have added extra tanks, did the addition of this extra fuel weight noticeably affect your gas mileage?