Welcome to the forum.
My guess would be around 6. You do not buy a motor home for great mileage. Poor gas mileage is part of the hobby/sport/lifestyle, whatever you want to call it.
It's a great way to travel and see the country. Enjoy.
I don't know if you are aware, but RV tires age out before the wear out.
Just because they look good and have plenty of tread, doesn't mean they are safe.
Ask questions, there is a lot of info available on the forum.