You will not find a better diesel engine than the 5.9 Cummins. Period. Case closed. End of story.
That being said, you might see the difference between 8 - 9 on a gas motorhome and 10 - 11 on the 5.9 diesel (speaking from first-hand experience with both), HOWEVER, factor in the higher price of the diesel fuel. Around here I've seen it anywhere from 20 cents a gallon to 80 cents a gallon higher than regular unleaded. Percentage-wise at best it's a wash, but I wouldn't let fuel economy be the sole deciding factor.
