WOW! The Pic came out huge. Sorry.
For some reason your post didn't come back to me. Make sure you use the Reply to Question at the bottom of the post to return your answer.
A decrease in gas mileage can be contributed to a bad coolant engine temperature sensor.
Since you have been getting your fuel at BJs for 10 years, I wouldn't think that is the problem. I have not seen any TSB etc on problems with the alcohol etc. Many of the vehicles now days are so sensitive it wouldn't take much to put them out of running order. Has anyone else that you know of had the same problems from BJs?
I suggest you check the ECT.
If you need more info from me, please make sure you let me know your exact model, is your vehicle a Pickup or Blazer etc. Also is your engine size a 4.3L and the 8th digit of your VIN X?
Sunday, November 26th, 2017 AT 9:08 PM