A 55 gallon tank can get you to and from a campground without having to fill up while towing a massive trailer. A 55 gallon tank can get you 385-550 miles. That's a good 7+ hour round trip with no fuel stop.
MPG is usually asked about because the person is wishful thinking. They would like to believe that they won't put out a lot for fuel. Reality is they'll put out a lot on fuel.
Driving speed is asked because folks don't like going slow. Folks get irritated when they get stuck behind a slow moving RV and when they decide to get an RV, they don't want to be that slow driver.