Advice needed 1st time shopper for gmc 1 ton or 3/4 ton truck

[KC_KORRAL]

Shopping for my very 1st 1 or 3/4 ton GMC truck. Don't know a thing about GMC Trucks ADVICE NEEDED BADLY. I currently own a 2008 Ford F350 6.4L V8 SuperDuty PowerStroke Diesel 1 Ton Duelly and am looking to get away from Ford's and all their major problems. I need a truck with a tow package to pull my 40 ft 5th Wheel RV, RV GVW is 17,500 lbs, hopefully get at least 15mpg and doesn't require a whole lot of major repairs. Thinking of year models anywhere from 2005 to 2011. Will some one please advise on a year model and which engine that would best suits my needs?

 

[Chzzzit]

I had a 2006 GMC 2500HD Duramax with 3.73 gears and it got on average 20mpg empty. Towing or loaded it got about 15mpg. This truck would pull a 20,000lbs trailer without issue or loss of power.
Currently I drive a 2011 GMC 3500HD gas 6.0 with 4.10 gears. Both trucks are crew cab 8' bed 4x4s. The 2011 gets 12 mpg empty and 6-8 mpg towing. It struggles to tow 10,000lbs.
Use this information as you wish. I will buy a diesel next time.