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. 

WE have a 2003 Toyota Mini-van.
On our recent 3800 mile trip from Seattle to AZ and return, I used cruise control on the flat stretches.
Worked fine.
I turned off overdrive whenever the transmission downshifted.
The Toyota's V-6 was a champ. Handled the 6-7 mile long mountain climbs very well.
I drove conservatively, but ended up passing semis on the long climbs, just because there was no reason to slow down.

We've got a somewhat modified 182G and tow with a newer Acura MDX.  I don't use cruise control in the hills, but I've found that the adaptive cruise control is awesome on the flats... especially in heavier traffic.  It uses radar to range the vehicle ahead of you and maintain a constant distance.  What a difference it makes on HW 5 in California.  I set the car for 57 Mph, and just steer.

We only use c.c in the open country when the roads are flat to gently hilly, otherwise we use the pedal.  As others have already stated, the engine is constantly searching for the correct speed and the tranny is constantly shifting as well to accomadate the engine rpm's.  

On our very first trip - home from picking up the POD - I learned not to use CC. The engine would kick down to 3rd gear all the time and I saw I was getting about 7.5 MPG. The LR3 has a manual shift option. I quickly learned to use that and mostly keep the beats in 5th gear, in 4th up some hills and occasionally in 3rd when needed. At a highway speed between 60 and 65 I now get 12.2 - 12.4 MPG. My tach is my shift guide.