Well, it is not necessarily that your O2 sensors have gone bad...otherwise, a code would have been thrown...it is more of they (you have more than one) have gotten 'lazy'...slow to respond to changes which could cause your vehicle to run either lean or rich. In your case, I would say rich because of the lower MPG.
In another forum I often browse through, this is well discussed cause for lower MPGs.
Also, are you buying gas from the same gas stations? If yes, have they changed their formulations...have they gone from pure gasoline to E10/E15? That would definitely affect you MPG as ethanol, while cleaner, has less energy density.
Many gas stations in my area use to sell pure gasoline, but now (because of federal mandate) sell E10/E15.
Oh, just some advice and not trying to be offensive here...in your post(s)...the way you word things and the language...you do come across as a punk. It may be that your are venting, not sure but I would tone it down a bit.
Now, having said that...I DO NOT agree with some of the others here in the way they responded to your post. It shows a lack of maturity.