I have driven small diesels in vans for quite a few years. The first one was a Fiat in 1996, a few times a year, several different ones.
They are gutless, but they get the job done. I remember driving a Fiat Duccatto in Italy with 14 passengers. We were all young guys, and it was floored more time than not. It got us there and back, between 50 and 60 miles per hour, going through a lot of different landscapes.
I don't believe it will give you that much better millage. A 4 cyl running full bore or a 6 cyl running at 90% won't make that much of a difference. I have driven 1.1L diesel cars compared to 1.3 or 1.5, when used in similar use, there is not too much difference in fuel consumption.
I respect if you want to drive a MB, but please don't say it is because of the fuel economy. It will be ALWAYS more expensive to drive the little MB based RV than a hungry GM 8.1 or Ford V10. Investment and maintenance far offsets the savings on gas.
You want to save money? Buy one of the new Dodge (Fiat) based motorhomes. They will be cheaper (gas) and you will have service centers everywhere.
Just my $0.02
