I'd have to say that if RV'ing is your thing, but you are concerned about MPG's on said rig... Well... RV'ing might not be for you..
I remember the $4+/gallon days back a few years ago.. Yeah, it hurt the wallet... But I still did it because I wanted to camp in my RV at the time and I was getting at best 10 mpg..
That's about the best figure you can go with on mpg's for any type of RV.. Gas, diesel, whatever you have ain't gonna get 20 mpg moving a big A RV down the road..
For those that claim over that, good for you! It just ain't what we all see, so good for you. Keep on keeping on... Oh, who is the so called "President" (past or present) does not control the world... No matter who it is.. :)
Good luck! Mitch
