Need some help
 

I have a2008 F-350 ,5.4 v8. When I first got the truck it was getting 11.5 mpg.Drove it around about a month and headed to Maine from Virginia Beach.Left from here with about 3000 miles on it .Well the miles per gallon continued to improve all the way up to 16 mpg. Now I have over 7,000 miles on it and stat back to Va.The mpg starts to go down .Now it only gets 8.5 mpg. It's the same miles up and back. Can any one tell me what is wrong ? I'v taking it to the dealer they say nothing wrong with it.I even put a chip in it and that did nothing at all. Any ideas ?