FWIW we just completed a 6 week, 6,500 mile trip up through the northwest. Fabulous countryside and met many fine people. Mostly I drove at about 65 - 67 mph but faced lots of strong and gusty winds and lots of high mountain passes. We used 913 gallons of gas for an overall average of 7.1 mpg. The highest cost for gas was in California at $4.57 and the lowest was in Missouri for $2.39.
My coach is a 32 foot Class A on a 2015 F53 with an 18,000 lb GVWR. It has the 5 speed transmission and the 5 Star tune. I did the front and rear CHF, front Sumo springs and the Roadmaster steering stabilizer. The handling was fine throughout the trip even when big trucks were blowing by at 80 mph and/or with strong gusty winds. I like my 5 Star tune but really only for revised shift points. As much improved as it is it wasnít enough at high altitudes and heavy load conditions. It still has a pronounced tendency to downshift into 3rd gear even when 4th would be fine. When this happens at even 60 or 65 mph it gets rpm and noise up in the stratosphere. It was not so much of an annoyance once we got back down to lower elevations and werenít facing long or steep grades.
My coach (2016 Fleetwood Storm) worked great save for a cab air conditioner issue (hose wore through for rubbing on the frame) and a large metal heat shield that fell off the chassis frame on the interstate in Montana.
Great trip through 13 states. It brings our total to 37 states in our 3 years of RVing. Love the lifestyle!