We bought our 2011, 3500 4x4 Cummins Ram Auto (stock) 6 weeks ago and have done almost 14000kms in it, over half that towing and here's our results, 90% of the time using cruise, except towing on up grades I switch off cruise.
Without 5th wheel:
55-60mph open road 19-21mpg
Towing 5er +- 8500lbs:
55-60mph open road 11-13mpg
With head wind 55-60mph open road 9mpg
We have crossed the rockies and done some steep and twisties both with and without the 5th wheel and the mpg difference is almost nothing maybee half a gallon worse.
On all the decents we used the engine brake, barely if ever did we use the normal brakes.