Where you need to start is at the O2 sensors and spark plugs, plus a thorough cleaning of the Mass Air Flow (MAF) sensor and upper intake tract.
While looking at the scheduled maintenance guide for your vehicle, I'm not seeing these things mentioned (though I did a cursory glance). It is common practice for original O2 sensors and spark plugs to last for 100k miles on a vehicle. I noticed you didn't mention you changed them, so am mentioning. What happens is exactly as you describe as these items become older and well used ... that being gas mileage slowly goes down. The O2 sensors become what is known as lazy. Basically, they don't work as efficiently so fuel mileage suffers. As spark plugs become old, they to do not burn fuel as well as they used to. The gaps become wider and wider until they fail to allow spark to pass. Changing these two things out (all spark plugs, plus front and rear O2s) along with a computer reset should help increase your fuel mileage back to where you are used to seeing it.
While you are getting them changed, getting the upper intake tract and sensors which affect engine efficiency cleaned will help as well. I would get this type of cleaning done every 50k to help with the optimal running of the vehicle.
Also, I would change out the air filter if this is something which you have not done. This can cause a lack of air flow into the engine, causing you to push further on the gas to get the same amount of power out of the engine. This will cause a reduction in gas mileage.
