Hi,
While I normally get about 7 MPG with my 460" V8 1997 Bounder, I did get a huge surprise when I was driving only 55 and started running Chevron 91 octane gas. I got 8.8 MPG. I normally tow a 3,900 pound Ford Edge, but that trip I was towing a 1,800 pound Manx dune buggy. Yet still I had the RV overloaded with all my stuff, I was moving from CA to Oregon.
If you have fuel injection, try running higher octane gas, you should see a mileage increase that is about 10% to 15%. So if the fuel cost is only 4% more than 87 octane, you will see an overall cost per mile go down.
Fred.