I log all our gas. I am at 26k miles averaging about 6.4 MPG, that is towing, not towing, heavy load, light load, 65-70 and 55-60. I can look at the current tank, 4 tank, 10 tank and lifetime averages. Our best has been taking it 55-60 without the tow, a bit over 7.
I have a scan gauge II, I like it as it gives me an idea of how I am doing, but it is nowhere near accurate. Last trip it reads 7.0, I filled up and 6.35 is what I got. now that could have a little bit to do with how full is full, but that is a consistent difference. I will read up on the scan gauge to see if I can 'adjust' it.
We did 'chip' the coach and that made a difference, not so much in mileage, if any, but in the pleasure of driving. The shifts are much better. Rig stays in gear longer and down shifts in smaller steps.
I would love to have a 35 foot 20,000 pound gas rig towing a jeep that gets 8-9mpg people talk about. I don't think it is happening. I could however I think make a suggestion to the builders that might make a difference. now that we have flat screen TVs they could rake the front end back a foot and a half like before TV was the big thing and they started sticking deep Tube type TVS up over the front end. I would bet that sloping the front end would cut air resistance considerably and likely increase mileage.