11 mpg towing, is not that bad.
Mine gets 13-14 mpg solo city/little hiway and 8 mpg towing a 13K 5th Wheel toy no matter what the conditions are.
When towing I`m in towhaul and manual shift and pretty much go no higher that 5th gear.
My opinion you don`t want to tow down the hiway below 1800 rpm or you can chance lugging which would be bad.
I try to maintain 1800-2000rpms on the hiway towing in towhaul.
On the tuner I would suggest tow mode that is what that setting is for,unless the standard is not a much higher setting.
And have a egt gauge to monitor you temps,the egt gauge will tell you what setting will give you safe egt temps. Plus you`ll know to get out of it,to keep from to high egt`s.
2007.5 GMC Sierra 3500 BLACK SRW C.C. 4X4 LMM/Allison,long bed,Loaded,Westach EGT/Boost pass side, Edge Insight CTS EGT driverside, DIY stock airbox mod, aFe proguard 7 OEM airfilter. Back-up sensors + Back-up camera tied into the navagation screen, Redi Lift leveling kit, Cognito UCA's, Pitman Idler arm support, AMP Power steps, Firestone Air bags with controller in cab.