Mine too. It started declining at about 101k early last summer. No codes. I've replaced plugs, EGR, oil, trans fluid, air filter, always run synthetic fluids since 8k. Timing belt, water pump, coolant, belts, all replaced at 80k. Currently have 106k on it.
What is the mpg? By declining what are you using to determine that? Guesswork doesn't mean anything. If the O2 sensors were deteriorating enough to cause noticeable decline they would be throwing a code as all functions of it are monitored closely by the PCM. It can also be checked with an obd2 scanner as well as most of the other sensors on the engine.
You don't need laser thermometer to see if a brake is dragging e.g. frozen caliper or stuck parking brake but it is a good excuse to get the thermometer. I picked one up from HF it is cheap but does not give consistent results.
+1 to check the engine temperature. The dash guage is useless for diagnosis. Many OBDII readers can also read the engine temperature. That's the value the PCM gets. Mine was cool by only 5 F degrees, and it affected fuel mileage. A good OBDII reader is a must-have.
Also, check the air filter. And try changing fuel brands. They may be adding more ethanol, which lowers fuel economy. Even try a tank or two of PetroCan 91 - it has 0% ethanol.
You could also get an emissions test. That would tell you if the fuel and ignition systems are okay. If so, then it may be something mechanical.