Your mpg when towing is pretty typical for any gasser tow vehicle pulling a 11'-12' tall brick down the road. When truck manufacturers tout there towing capacities they are using boats or other more aerodynamic loads and not 12' tall RVs with tongue or pin weights that quickly exceed payload capacity well before towing capacity is reached. Driving slower is the only sure way to increase towing mpg, but I don't fret about it much as it is just the price we all have to pay to enjoy the RVing lifestyle.