Gas mileage dropping on my 98 5.7L

[Tom]

My 98 has always been excellent with the gas mileage. When I first got it about 8yrs ago at 75,000 miles, I would do about 17mpg city and 22mpg freeway. The city has slowly dropped till it averaged 15, the freeway I was still managing 22 [remember its all flat out here, no hills anywhere]. Over the last year, my freeway has been falling as well. I'm down to like 17 freeway, which means something isn't right. My last tank average was only like 14mpg.

I maintain the crap out of this truck. Fuel filter, wires, cap+rotor, wheel bearings, brakes, trans fluid, spark plugs, air filter, u-joints, intake gaskets and fuel pump are all new or recently new. The truck still runs great. Pulled all the plugs and they are evenly worn and look good. Checked compression on a cylinder from each bank just cause and they were excellent. My brake booster went out about a month ago and leaked vaccum so I was hoping that was the cause. New booster and mileage is still getting worse. So now I'm left guessing what could cause this. Cat convertors? O2 sensors? I would think they would trip a CEL eventually though. What can I check at this point?