1989 Nissan Truck fuel economy

1989 Nissan Hardbody 2WD mileage: 267,000. Help me Bob and Ken! I am having a TERRIBLE gas mileage problem. In the last 8 months, my fuel economy has dropped from 16 mpg to 12 mpg! I have spent over $600 having the problem diagnosed to no avail. Repairs that were made include Adjust throttle linkage, Scoped and set engine, throttle body gasket repair, timing adjusted, tune-up, new plugs, new O2 sensor, new catalytic converter, vacuum lines tested, emissions tested, and exhaust system check and fuel injector cleaning. After putting the truck on 2 different diagnostic computers, at 2 different places, 2 different mechanics have stated the engine was in great shape to have so many miles. I know that with so many miles that I'm not going to get the same gas mileage as I did when I bought it (24 mpg), BUT 12 MPG IS CRAZY! Please see what you guys can come up with.