Well Sir,
Without sounding too negative here. If you're going to drive ANY type of RV, then gas mileage is not in the picture. Some "supposedly" get in the 10s, some in the 9s, but, by far, the average hovers between 6-8. They're big, they're heavy, they're usually pulling something that's not light, they're built like a Kleenex Box on wheels so there's absolutely "0" aerodynamics.
We ran two class Cs for several years. One was a 23' Dodge "Four Star" with the 440 and it got somewhere in the 7s range. The next one was a 27' Class C Ford "Ultra" with the 460 and it got in the low to mid 7s. We then moved to a '99 Fleetwood Bounder 34V with the V-10. It got in the low to mid 7s.
We're now in a 2004 Itasca Horizon 36GD with the C-7 330 HP CAT and 6sp Allison MH3000 trans, and it hovers between 6.5 and around 7.5-8 depending on how much down hill I get to drive on. All of our motor homes never moved an inch without something hooked to the back of them so, again, gas mileage was, is, and never will be a factor. It is what it is.
And, as far as what type of gas, we've been using regular for all my driving years. 99.999999% of the NORMAL, everyday, common vehicles produced over the last several years, I'm talking 20 years, run on regular gas. Some of the manufacturers "recommend" the use of Premium but, the engines run just fine on regular.
All of he gas coaches we had ran on regular and, we live in CA where the lowest octane is 87. But, when we entered states like UT, where 85 octane was available, it went into the tank too. Absolutely no running problems when running that fuel.
So, unless you're going to run one of those newer Winnes with the Mercedes diesels in it, don't plan on getting too much mileage, especially if you're going to tow something.
Again, not trying to be negative, just being real.
scott
Scott and Karla SDFD RETIRED2004 Itasca Horizon, 36GD Slate Blue 330 CAT
2011 GMC Sierra 1500 Ext Cab 4x4 Toad
2008 Caliente Red LVL II GL 1800 Goldwing KI60ND