Ours is a 2004.5 Chevy D/A crew cab LB 4X4 and 13 to 13.5 mpg has always been the norm pulling our scaled total weight 12,840 lb Carriage CarriLite 5th wheel hand calculated. The DIC has been accurate within approx 1/10 mpg over the a little more than 140,000 miles the truhs been driven so far. Ordered and owned the truck since new and keep a log of all fuel, mileage, and maintenance costs so we know. We tow at 58-62 mph on Interstates and the speed limit posted on other roads. Truck is only driven for RV use. With our 11'4" very heavy Lance truck camper on instead of towing the CarriLite 5th wheel, we have averaged 13.93 mpg over the 71,986 miles of hauling it alone and 13.38 mpg average with our 2150 lb boat/trailer behind.
Speed and driving habits make a very big difference as does which vehicle brand/specs/efficiency is driven. Some vehicles are known and a proven track record to have either higher or lower average mpg's. The various tests, shootout comparisons, and consumer etc reports have been very accurate and constant in their numbers for so many years now.
Just because a person does not get as good of fuel economy with their particular vehicle or rig as others doesn't mean or have any bearing on what others are getting with their vehicle or rigs. You had the right to make your own decisions and therefore be the owner of the results of your decision whether it's positive or negative!