Part of your different mileage is that yours is a 3/4 ton. The gearing is different, and the engine fuel injection map and transmission shift points may be different as well. The one pictured there is a 1/2 ton. Also, how much towing of that 2-ton horse trailer do you do on downtown surface streets vis a vis highway towing?
Do you own that model? Are you certain it is a 1/2 ton?