We have an '08 Winne on an '07 chassis. No problems at all. Just normal maintenance, which is no more costly than other brands, with the exception of oil changes. The MB takes 13 quarts of a specific synthetic oil, so you will pay a bit more than you would when changing other brands. Then again, you only have to change every 10k miles, so it's a small price to pay. You can easily order what you need for normal regular maintenance through the internet.
As for fuel mileage -- yes, you can't beat a Sprinter, but they do cost more to buy. I tell people *don't* buy a Sprinter-based MH merely because of fuel mileage. Instead, consider it if you want a small MH that rides and handles better than anything else out there, while making less cabin noise/heat in a cabin that is much more expansive in all directions than the others.