I used to fill my work truck up at Shell stations all the time, and got 10 MPG, so I figured that the 30 gallon fill was good for 300 miles and 33 gallons for 330. Once I was not near a Shell, and used Cheveron. I was getting 11 MPG on Cheveron gas, and Mobil. Shell and Arco only gave me 10 MPG. This was a E-250 van.
You might keep a log and start buying only Cheveron and Mobil.
As a side note, I bought 91 octane gas for my trip from San Diego to Oregon in 2013. I was getting 8.5 MPG while running 55 MPH. Normally I was only getting 7 mpg, while running 65 MPH. So the improved octane and slower speed really paid off in lower fuel costs! For a 1,100 mile drive, that is more than a few gallons. The extra 5% in fuel cost per gallon saved me about 10% in fuel used. Slowing down helped fuel use too.
I think that you will get about the same MPG with the slightly longer class C as the older one. It will be slightly heavier, but also have the new 5 speed automatic transmission, so it should get better mileage if all things (weight and height wise) where equal. Of course slowing to 55 MPH will push a lot less wind out of the way, improving mileage substantially!
Changing to 91 octane will help the MPG of any fuel injected vehicle. Try it and see! The anti-knock sensor will see that the fuel is better, and can run the car leaner, and advance the spark for better HP per gallon of fuel.
