We travell at 55 miles per hour on a "C" and we normally get between 9 to 10 miles per gallon on a FORD V-10 24 foot C, maybe in a "B" you might get better milage but you will not have the same room and or the same batheroom space or the galley space or storage.
Before you purchase anything go and take a look at them and see if it really is worth changing the RV, pretend to use the toilet, the sower, the galley, siting on the chairs to have a meal (when the weather is bad), etc.
Do not let your emotions take over the "NEW" and really see the reallity of the actual space, some units are Diesel powered and today I see the price being higher for Diesel than for regular gas.
navegator