we are one our 1st long trip with a winnebago aspect B+. I have been haphazardly figuring milage and it seems to be in the 8-8.5 range. Was down to 7's in texas hill country with winds and got into the 8's still in hll country w/o head or quartering headwinds. Left dallas and had 9.2 in murfreesboro ark. 9.19 in little rock. seems to be a variance in how the veh performs with again speed. low were 65 mph or so, mid was around 60, latest back between 63-65. in ark we were on back roads no traffic perse so no real changing of speeds so all was fairly constant. some slow driving due to where we were at and then going to but not a lot. watching my scan guage for the instantaneous seems to indicate more fuel use at low settings vs the fuel use at 60+. Engine out of its torques/hp levels at low speeds thus more fuel use (less mileage).
I have noticed as the engine ages (more miles) it seems to be better in response to throttle inputs and I am also using a 5 star tune. will know in another couple of thousand we have left to cover until we get home.
BTW I drove a small chevy school bus last year 12 passenger 3500, best milage it ever got was 12 per its computer. I weighed a lot less than ANY rv would and had the 350 engine, 6 speed auto. yes lot of stop go but also had at least 80 miles a day interstate.