There are lots of factors in "fuel economy" ... shortest route, wind conditions, changes in elevation, speed of travel to name just four.
I think of RV travel as being a way to see the sites of the country ... if you are thinking economy then, in my way of think, you should buy a Toyota Corolla (or equivalent) and stay in cheap hotels that provide continetal breakfast ... buy bread and bologna in the supermarket and eat lunch out of your car ... and have dinner at Golden Corral.
RVing is a lifestyle ... the journey is to be enjoyed ... you have your clothes with you (most likely in a drawer), you know who slept in the bed last night, you have refrigerator when you can keep milk, beer and an ice cream bar ... fuel economy does not fit into the equation in my way of thinking.
My biggest expense in owning my motorhome is the depreciation, insurance, repairs, and the monthly payments ... together they make the annual fuel bill look like a pitance. That is my $0.02 worth ...
2004 Winnebago Journey 39W - 2012 Buick Enclave
Present @home Home: Oshkosh, WI
We call our rig "Ernie the Journey"