They have been putting ethanol is gas for years, change gas stations, some stations i get 12mpg others i can get 23mpg. marathon is by far the worst so far. every time you get fuel it is different, it always has a different specific gravity, and different SG's call for a different stoich ratio.when there is more ethanol and additives, it takes more fuel to get 14.7:1 ratio that the ECU shoots for,(which is for pure gasoline that you cant buy now) Its impossible to tell anymore.
You might have a slightly off o2 sensor thats causing it to run slightly rich, but its still telling the ECU that its in range which is why theres no CEL yet.
The fix for my truck when the mileage starts to go downhill is to go run the F out of it a few times up and down the highway and all kinds of **** blows out of the exhaust, then it gets alot better for a while.

 

MY 09 Is stock 5.3 4x4 ext/cab 6000 miles and I get tops 15.2 hwy and 13 around town and I drive slow and easy on the pedal I dont think I will ever get 18.....

 

how about a simple plug change? they might b fouled out.