1986 Itasca 454
My 26' Mobil Traveler got 7-12 depending on the airspeed
This was a 6.2 diesel, driving steady 55 it regularly got 11-12.
Driving 70-75 it got 7. Headwind matters, traffic matters & speed makes a large difference in economy with all that frontal area.
Get a good stereo & enjoy the ride