If the cruise control works on the old girl, set it at 55 for moderate to somewhat hilly roads for best fuel economy. One you notice it having to pull the pedal down a lot to keep speed, turn it off and keep a fairly gentle, steady foot on the gas, letting the speed drop to 45 (highway minimum in most states) so you aren't running it as wide open and guzzling more gas. Let the cars pass and enjoy the ride, and save the fuel. You won't lose much time in the long run, especially if you are relaying rested drivers. Longer, slower steady speed runs save more time than bursts of faster speeds with long breaks between.
Eric & Holly + 2 kitties from New Hampshire
1992 Winnebago Adventurer 32'