Since most Class C motorhomes are kindof large as compared to pickups, family sedans and SUVs ... we cruise like a truck in our RV at "truck speeds" on the open road.
We travel at 55-60 MPH on highways and freeways on trips - the same as the legal speed is for trucks in many parts of the country. We do this in truck lanes as much as possible, however, so as to not irritate the speed demons around us.
Traveling at these modest cruising speeds is not only a very safe way to drive a 12,000 lb. vehicle, but it also gets us in the neighborhood of 10+ MPG fuel economy with our Ford V10 Class C motorhome. Speed kills gas mileage (and sometimes ... people) when moving a big box down the road.