Towing my 5th wheel (15000 lbs) from BC Canada to greater Phoenix and back twice, I averaged 11.3 mpg (US gallon).
For your info, in 2011 GM upgraded the frame, suspension brakes and steering which greatly upped the GVWR of the 3500 trucks from 9900 lbs to 11500 lbs, however they do weigh more.
An added plus for 2011 and up is the factory exhaust brake and increased HP and Torgue.
The CCC on my 2011 SRW Crew cab, is within 50 lbs of my old 2006 GMC Crew cab dually, both equivalent models.
If I seem GM biased, I am, and have no personal experience with Ford or Ram, so won't comment on them.
