If you buy a 2017 and later Motorhome with a Ford V 10, you'll have the 6 speed transmission and your mileage will be superior to many older rigs. My 31 foot Jayco gets 9.9 mpg at 60 mph. Rigs are advertised as how many they can sleep. The bigger problem is how many they can walk. If your use to a respectable size trailer, it's doubtful you'll be happy in a small B. Take Lazy Daze for example. They generally know what pleases people, and they only sell three sizes; 24,27 and 31 foot. I've seen a number of folks here say 27 is a good sweet spot for size versus maneuverability.