2001 Lincoln LS Sudden decrease in mpg

This vehicle seems to run fine with no driveability problems. However, mpg suddenly has dropped 2-5 mpg with same drivers, same routes, same fuel, same driving habits. This appears to have taken place a month or two after it was placed in storage for 6 moths. Prior to storage STABL fuel stabilizer was put in the gas tank & circulated over 50 miles. The car was purchased as a Lincoln dealer loaner with only 10,000 miles and has always followed the dealer maintenance schedule. When the dealer drove it 12 miles with an on-board diagnostic computer, it showed a normal 20 mpg and no problem with any system. I had a repair shop put in a 3 part "industrial strength" fuel injection & gas cleaning system with no improvement in mpg. Going forward this can will be driven 6000-8000 miles annually and be in storage 6 months at a time. Any cost effective solution to this unsatisfactory increase in fuel consumption.