Where speed limits are 75 and higher, I am more comfortable at 65-70 mph in my motorhome. Much over that speed, it might be necessary to downshift out of overdrive to get enough horsepower to move faster. That's crazy in a motorhome, although the last four cars I've had, top speed was available only in a gear lower than top gear, something useful only for a high speed overtake.
I find that when speed limits are this high, something over 70 mph, most of the traffic is not driving over the speed limit, or even at the speed limit, which is the way speed limits are supposed to work. FWIW, the 75 MPH in Oklahoma is a zero tolerance speed limit, in a state that routinely gives 5-10 mph slack over the official "no signs" 55 mph limit.
The idea that the speed limit is a minimum, and traffic should travel 5-10-15 mph over, is a mindset from cultures where speed limits have been set artificially low, on the premise that every one will exceed them. Vicious circle. But if you are coming from the East to high speed limit states, leave your ideas about tolerances at home.
Texas, I've heard the rule is "drive as fast as you can afford." Visiting Texas often, that seems familiar, but doesn't mean I speed in the motorhome. Texas and Missouri drivers thinking 75 MPH means 80-85 are a revenue source on Oklahoma turnpikes, and on some of our 70 MPH two and four lane highways.
"Happy speed" for my Class C seems to be around 60-62 mph no wind, might drop as low as 50-55 in a bad headwind (where it has to drop out of overdrive to push past 60), but then again a 15 MPH tailwind makes 70 mph an easy cruise.
Tom Test
Itasca Spirit 29B