What a lot of people forget is that manufacturers install diesel engines according to weight. If you have a heavy coach, you are going to get a five or six hundred horsepower engine. If you get a light weight coach, lets say around twenty thousand pounds, you are going to get a small diesel like a 5.9 Cummins. But you will not see from the factory a light coach with a big monster diesel. It just does not happen.
So diesel engines on the average will get anywhere from six to nine miles per gallon of diesel.
Now there were some two cycle DD engines like my buddy has on a Bluebird which weigh's 45K lbs and it only gets five miles per gallon of diesel.
Larger diesel engines are installed to move more weight. So it all evens out in the end.