Hey look at it this way: you found something new and it corroborates with what my tuner said, that to get that drastic of a drop in mpg you had to have some sensors go bad.
Now the question is what to do. If all three sensors are indeed bad, what caused it? Electric surge? Drop in water and they corroded? Shitty luck? I say this because you may be better off replacing all three at the same time instead of one by one in case a bad sensor has, somehow, caused all this in the other sensors.
You are to the point now where it might be better to buy/borrow a PC so you can run TuneECU. Or, better yet, can you download your tune and any diagnostic stuff and send it to me? I'm wondering if you can download a diagnostic file that Wheeler, or anyone here, may be able to use to confirm the sensors are bad. If you can get a second opinion then I dare say you just solved the mystery.