Okay, I have tried 3 more tanks of various gas brands. Highway MPG now varies from 24 to 25. I am not using an onboard computer to do MPG. I reset trip odometer each time I fill up and make my own computations. I do not top up the tank, but there is not enough room to make that much difference, and I have never topped off tank, anyway. No one else drives the car and it has not been worked on at all since the drop started, except to have oil and filter changed AFTER the drop occurred. Now I have had 6 fills of different brands, and the mileage is still way low. Tire pressure has been checked and is not only correct, but also at the same level as when I was getting much higher MPG. There is no CEL on; I would have long since gone to the shop if there had been, thank you! Air filter is fine, been checked. No dusty area driving.
I am inclined to have the thermostat and coolant temp sensor checked as Tester suggested. Many thanks for all the good ideas.