The hills out west will not allow you to get 7-7.5 unless you are willing to slow down.
Speed equates to a headwind and with headwinds or speed the mileage goes in the dumper. Add in hills and no matter how much fuel you save going downhill, it never makes up for what you burn going up. Exactly how much impact depends solely on how fast you are trying to go. I have a retarder, so on my downhill runs I dropped it down to 4th and held the speed to 55 because the steeper grades would have been tough on the brakes. That may have impacted my mileage also. Going up those same hills in the opposite direction I was lucky to see 55 so you have some idea of the grades that were impacting my mileage.
Your OTR will take a few tenths of your MPG, so the bottom line is when we travel in the west with our 46,600 pound coach towing our H2 Hummer we weight 53,000 plus and are lucky if we can get 7 MPG. When we went west on our last trip not towing, we got 7.3 for the approximately 4000 mile round trip.
As much as I hate paying for fuel, I just accept the fact that whatever it is there is not much I can do about it, and not using the coach as much as we want to compensate doesn't make sense.