"Fill ups” can vary quite a bit due to MH tilt at the station, gas nozzle shutoff adjustments, how quick you want to get done, wind, etc. Multiple fillings are the only true test of MPG. In the 52,000 miles I’ve driven my 31 foot 1998 Itasca over a 1999 Ford F53 V10 (with 105,000 miles on it) its averaged 8.14 MPG with a per tank high of over 12 MPG and low of under 5 MPG. As you, my normal speeds are 63 MPH on interstates and 56 MPH on other roads. The overall average MPG has dropped over past 52,000 miles from 8.44 to my current 8.14.
Dave