I do not own a class A yet, but looking for one and I too did a lot of reading on MPG. As far as I can remember, it is allways between 7 and 8.5. The odd one will be low 6 / high 9. This seems to be the norm from unbiased RV gaz or diesel onwner, especially one that have a camper over 36 foot.
When I ask a private seller, or fellow campers, it is more like in the 10 to 12....I do have some doubt from canadian RV owner when they calculate, our imperial gallons are bigger than US gallon and some just do not know how to figure this out.
I think it depends also a lot on your driving habbit. I come from Canada, and the price of fuel being over $5.17 dollars right now, I am sure my habbit are much more gentle than when I drive in the US. I would think that you can get 10 to 12 with a smaller gaz RV, like a 29 to 32 foot one, if you are gentle on the pedal. But with a smaller 5.9 pre 2008 cummins, and a 32 to 36 foot RV, I believe you will get 10 to 12, even in the 13 if your driving habbit are like "Fuel is at 5.16 a gallon".
I went on a 12,000 miles trip with my son, keeping between 50 and 60 MPH, and no major head wind all the way through our thip. He has a 2001 5.9 cummins with a slide in camper. He got 16.5 MPG (US) hand calculated while I got average 13 with my Ford 6.0 Diesel with a bit bigger truck camper. These 5.9 have so much torque at low RPM, I was amazed when he was passing me going uphill, you could hear the engine like it was idling while pushing this little camper, not even shifting. If he was at the right RPM/MPH, he was kepping his steady speed while I had to down shift and press on the pedal to keep up.
Good luck in your research.