When you tow a vehicle (toad, I assume), you need to think differently about mpg.
If your toad gets 20 mpg when driven, not towed, and your RV mileage goes from say 12 to 8 mpg when towing, you need to factor in the gas for the towed vehicle you didn't use.
Say you get 12 mpg on a 120 mile trip, then you tow a car that when driven gets 20 mpg, if you drove both to the location you'd use 10 gallons for the RV and 6 gallons for the car or a total of 16 gallons to travel 240 miles, giving you 15 mpg effectively for the two vehicles. If you towed the vehicle and got 8 mpg then you'd use 15 gallons, for both vehicles to get to the same place or the equivalent of 16 mpg for the two vehicles to travel 240 miles together.
2013 Mirada 29SE, I earned it!