We also have a a 2011 chev 5.3 /3.42 and when we picked up our 5TA I took a very brief trailering lesson next to Escape at the driving school, the guy I took the course from told me to always drive it in the tow - haul mode, which I do, and I get the same milages as you are getting- I do get better milage, more around 17 if I go under 60 mph and there isn't much wind.
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Min D
Chapi II
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'Have No Regrets'