Cruise Control While Towing?

I tow with a Chevy Silverado with auto-transmission and Tow-Haul.  On level runs at 55 mph with no head-winds I get about 12-13 mpg.  In hilly areas with Tow-Haul engaged, I average about 10 mpg.  In steep mountain areas I am lucky to get 6-8 mpg. The engine is pretty big...even stiff head winds only drop the mpgs by about 1 mpg.  Towing at Interstate speeds, 65-75 mph, my mileage drops 2-3 mpg at minimum.  The frontal area of any R-Pod is so large that even an increase of 10 mph will drastically cut your mileage.
One big difference is the type of fuel you use.  On my last run to Florida, my mileage dropped by about 15-20% where only ethanol was sold.  I bought non-ethanol gas if the price was within 20% of ethanol.