An Update. Buried above is a post I made about my V10 all of a sudden barely getting 3 or 4 MPG when I usually get 7. My regular guy blamed winter blend.
I took it to a chap I use when we're in the mountains. I told him my problem and he asked how many miles on it? I said 40K. He said: "O2 Sensors". He said they age and will keep "asking" for more fuel long after it isn't needed.
Yesterday was my first run since he changed 2 of them and I got a solid 7.3 average MPG for a 6 hour run and the engine just ran better. I'm a happy camper again. Hope this helps someone down the road.