My experiences with 34 ft Southwind 454 Chevy towing a HHR toad usaually running about 62-64 mph.
4,000 miles plus. Illinois to Texas to Arizona back to Texas and Illinois. Low of 6.7 high of 9.8. Overall average of 8.1. Varying wind and terrain conditions but no real mountains.
Most recent. 3,050 miles Illinois to Florida to Texas back to Illinois. High of 7.8 low of 5.6 overall 7.2. 5.6 was across Missouri with engine missing and driving like I stole it to get up the hills.
Since the last trip I have replaced plugs, plug wires, distributor cap, coil (everything ignition associated) as well as belts, hoses, coolant etc. Ran great on 90 mile trip home. Hopefully this winters trip will produce favorable gas mileage.