Using two methods to calculate, we averaged 9.5 MPG on our last trip to Florida, from Long Island, with no toad. It was about a half-gallon lower a year ago when we pulled the toad.
The two methods were 1-Scan Gauge II and 2-a manual calculation of mileage based on fuel consumption at each fill up. Manual calculation was slightly lower. Consistency is tough to maintain when filling up. I stopped when the pump "clicked" off at the handle the first time, at each fill up and used that amount of fuel for the calculation.
2004 motorhome on a 2003 Ford V-10 chassis. Kept the highway speed at just about 57 MPH on the highway per the Scan Gauge II.