I think I've heard horror stories on both the Sprinter and the Ford F-53/E450 chassis. I've also heard a lot of good things and there are a lot of people with problem-free rigs.
As another idea, there are two new major chassis designs coming to our shores in the next year: The Fiat Ducato/Dodge ProMaster, and the Ford Transit.
The Ducato has a lot of engineering to be motorhome friendly. How easy it is serviced, who knows. However, there are a lot of European upfitters which make small class Cs on this platform. Hymer and Globecar come to mind.
If you can bide your time, I'd see what people come up with on these new chassis. They will have their teething problems, but will be very economical when it comes to fuel consumption.