I drive a 2012 5.7 L with the "six" speed transmission in hilly central Connecticut and use regular 87 octane. The best mostly highway average I've seen is 20 miles per gallon on a long road trip. Right now, in the cold Winter air, I'm averaging only 14.2 miles per gallon in mostly local driving; during the Summer, I average 15.5 miles per gallon. When towing my ~2,000 pound boat on its single axle brakeless trailer, my average drops to as low as 12.0 miles per gallon, but the Jeep still drives almost like the trailer isn't even there.
I use Tow/Haul mode only in the hills; I turn it off on flats or long downhill grades.