Find gas without ethanol, that should help! I have hand written calcs on mileage and mpg from the first owner in 1984. He was getting 14-17mpg in the same van. I haven’t done better than 15. Most often closer to 12. There wasn’t ethanol back then, can’t think of any other changes that would make that big of a difference.
I can. How about 33 years of wear and tear on the drivetrain? Ethanol does have a lower stored energy than pure gasoline, but there are many reasons other than fuel mixture to explain a 3-5 MPG drop over 33 years. I wouldn't stress about finding ethanol free gasoline.