There are so many variables when figuring mileage. Head winds (or tail winds) hills, speed, traffic, stop lights, how fast you accelerate, etc.
I am taking it easy on a 2 lane highway just cruising along at 55 mph, and my Scan Gauge is showing a nice mpg. Then a couple of stop lights get me. They can suck down the gas mileage in a heart beat.
Cruising along at 65 on the interstate, the Scan Gauge is happy again, and then there is road construction or an accident where there is stop and go traffic at the bottleneck.
Hills....and we got plenty of those around here. I can watch my mileage go way south of 10 mpg going uphill, but then on the downhill side it tries to catch up again, but it seems never what it was to begin with.
And if you are counting overall mpg, you gotta figure in hooking up and unhooking the trailer.
I don't know exactly how many of those obstacles figured in to the OP post, but you have to consider those variables.
Also, I very seldom use cruise control while towing. With the rolling hills around here, I have to leave the tranny in 3rd (4 speed auto) if I use cruise control. In 4th, cruise control will make the transmission shift way too much. In 4th, with my right foot in command, I can loose a couple of mph on the uphills, and gain them back on the downhills without the transmission shifting. I seem to get much better fuel mileage that way.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263