Do not use good gas milage and "rv" in the same sentence. Just not going to happen.
If you only have a $10,000.00 budget, your probably looking at a class c with an older V-8, like the Ford 460. Great on power, but a real gas guzzler. Same goes for the older Chevy 454 V-8.
Now, I'm going to get really flamed for this, but even with a newer Ford V-10 or the newer Chevy V-8, your probably going to average 6.5 to 8 mpg. That's it, maybe a bit more or a bit less. Most who figure their milage use the "miles traveled divided by the # of gallons used" when they fill up. Too many variables to get an accurate #.
I have a Scanguage, which gets it's info directly from the vehicles computer, and it is accurate. Many who claim 10-11 mpg while running 60-65 mph would be shocked to find out what the real #'s are.
My class c gets between 6.2 and 8.1, and has averaged around 7.0 over the last several trips. And, going up a grade it will drop off to 3-4 mpg.
But, with that being said, you don't buy an rv and then worry about milage. Your getting into a different lifestyle, and for the enjoyment of the traveling and the destinations and sights you will see along the way.
If your going to constantly worry about milage and spending money for gas, then rv'ing is not for you, because it is a cost that is not going to go away, or get better with time.
Grumpy