MPG Changing All The Time

I have a 2006 Freightliner Columbia with 742,000 Miles on it. I just had the Overhead done because I have been having bad fuel mileage with it. Sometimes I will get 6.5 MPG with a 17,000 lb load and then other times with the same weight I will get 4.3 MPG, So I was told a Overhead would help. This has not helped, my MPG is all over the place once again.....Recently I noticed some black stuff in the fuel filter, I just went into the shop & the black stuff is gritty & now I noticed there was aluminum flakes along with the gritty black stuff in the fuel filter. Now my question is....What is the aluminum flakes and now where do I start? Do I change the fuel lines or do I flush the tanks out OR both. Also is there some other reason for the bad fuel millage that could be the issue? Thanks!​

 

Thanks for the reply! Fuel lines have no leaks.