Congratulations on your motor home.
We have also found that the speed at which we travel makes a big difference in gas mileage. We have a 6 cylinder and we get from 11 to 15 mpg. If we travel on the freeway at 60 to 65+ we see 11 mpg.
Last time out, a couple weeks back, we decided to take the side roads home which kept us at a max. of 50 mph. We were amazed to find that we used 15 mpg (first time in the two years we have had it). We also saw some beautiful farmland and went through neighboring communities that we seldom see. We have decided that in the future we will to try to find alternate routes to our destination, As we have been plotting future trips though, we are finding that the additional miles required by using an alternate route can use up the "saved" mpg. Always seems to be a balancing act.
Mike and Kathi