Not long after first finding RTA's forums and beginning to participate in same, I took a RoadTrip speed run from Raleigh, NC to Port Hueneme, CA (Ventura, a little above LA) with my US Navy son, for the purpose of co-driving his pickup from home, where it'd been while he was deployed, to his homeport, where he'd serve another year on active duty. It was essentially I-40 all the way.
I won't say exactly how long we took over how many days, etc, as I'm not particularly proud of it. But, we averaged 67 mph on the basis of "beginning of day to end of day", we economized on stops, combining the truck's 350 mile fuel range with bathroom stops and food, encountered essentially no urban-area or construction slow-downs, and enjoyed bluebird skies and dry highways the whole way. We were not towing, either, just a Chevy pickup with a young sailor's seabag and his Dad's luggage in the jumpseat. We ran a solid 80-85mph practically every step of the way while under way in order to "achieve" that 67mph beginning to end of day average. We touched 90 more than a few times.
Towing? If you average 50 mph from beginning to end of day, you're pushing very, very hard, IMHO.
Foy