2002 Tacoma fuel economy took a nosedive

My '02 4x4 Tacoma, 2.7 liter 4-cylinder gas engine, has always had very
predictable fuel economy...around 24-25 MPH highway and about 20 in town.
Late last fall, I parked it in a friend's barn for the winter while I was overseas.
He started it once a week and moved it around a bit so the tires wouldn't
flatten out. Now that I'm back, I've noticed my mileage has fallen by 25% or
more. Tires are inflated properly. I checked the entire air system to see if a
mouse or rat had set up house and all is clear. The "check engine" light has
not come on. A reputable maintenance/repair shop looked it over, read all
codes and found nothing. I'm perplexed.