My Bullydog has the econ/tow setting as the same. My Dodge is a 2006 4x4 with the H.O. Cummins. It will get 19-22 mpg depending on terrain. My truck seems to run better with a bit of a load on it. If I turn it up to the performance setting it will smoke the 38/12.50's just about as long as I hold it open. I never run it there cause the fuel mileage is terrible.
The biggest thing you could do to help your mileage would be to change gear ratio,4.10 is pretty low.
I ordered 4.33 gear sets in my truck with the six speed auto and I can pull anything I hook to.
We have a 41ft. Cyclone toy hauler that weighs in at 18000 lbs. Add all my wife's junk and two full size Harleys and 35 gallons of gas in the refueling station and I'm sure it comes in at or around 22000 lbs. and we pull that thing all over these mtns up here and out west. Fuel mileage really sucks but it will do it.