Here is my 2 cents,
I tow a Keystone Sprinter 5th wheel, 33', approx. 9900 GVW with my Chevy, 2009 2500HD, 6.0, 3:73 gears, 6 speed trans. My truck is a extended cab, 6' bed, 4 wheel drive. When I scaled the whole rig it weighed (with truck) 16,840#. Pretty much fully loaded.
My 6.0 gasser does ok pulling. You have to keep in mind that it is a gas engine and develops it's HP at high RPM's. I live just south of Kansas City. Most of my travels are in the Missouri Ozark region. Have pulled back east several times to include the Smokey Mountains. My MPG is very similar to the others with the SAME truck (8-9). If there is a head or cross wind it will dip to 7 mpg. My best was 11 in the Smokey Mountains!! One thing that I cannot to do is use the cruise control. And very seldom will pull in 6th gear unless the wind is right or there is a long down hill. Have a friend that was using the same truck except had a Duramax with the same weight trailer. No issues, pulled all day in 6th gear with the cruise and got around 12 to 14 mpg (and walked off from me)!
I have been flirting with replacing it with a diesel when the time comes. Mainly for the comfort of setting the cruise and just driving. Had an ankle injury years ago and holding the throttle for long periods of time causes pain. the other BIG positive is the engine braking. The 6.0 gasser doesn't work very good due to it being a gas. The exhaust brake on the diesels is way better.
Hope that helps. Feel free to PM me if you have any other questions.