I experienced the same thing with both of my diesel trucks a few months ago. Like you, I had been getting fuel at the same place for both trucks and mileage was down on both of them. One is a 07 6.7 Dodge and the other is a Mitsubishi Mighty Max that I had converted to an oil burner. Both are well maintained and nothing was wrong with either but the mileage was down about 2 to 3 MPG. I thought to myself the only thing that it could be was was the fuel. Turns out I was right. Started getting it from a different station in town and mileage is back to normal on both. This crap they call diesel isnt worth a dang anymore. It has no shelf life and dont last like it used to. Just like back in the day you used to be able to start an old diesel tractor or truck that had sitting for years with the same fuel and they would run as good as the day they were parked. Not anymore.