I have hauled a 27DSRL all over with a 2012 F150 E-boost and probably averaged 8, sometimes better, sometimes worse. Worst tank I remember was just under 7 mpg, best ever was just over 10 mpg. I usually always tow at 63-65 on the interstate, never more than 65 mph.
With our updated towing rig, 2016 Ram 3500 6.7/Aisin, mpg is better although I don't have enough time on it to know what I'll average. I've only put 600ish miles on it hitched up vs 12k with the F150/27DSRL.