One item overlooked in computing gas milage is that not all speedometers are accurate. If your speedometer is 5MPH fast at 60mph your miles to gallon is not 10, but9.1 mpg. I have owned 4 motorhomes ...
There's also the on-board generator which will throw off MPG calculations if used much. It'll also make MPG comparisons of gassers to diesel units with LP generators somewhat inaccurate, favoring the diesel.
Jim, "That cheese doesn't belong to you. It's nacho cheese!"