I've had M-B vehicles (not an RV) and they can be less than cheap to fix. They don't break down often so that offsets the expense.
Getting back to the original topic, it may be a case of M-B doing a little CYA. It may be that Mercedes hasn't researched bio fuels to it's satisfaction. When a friend was getting his Porsche ready for winter storage, he found that Porsche doesn't reccomend fuel stabilizers in their cars. Not that it will hurt the fuel systems but because they don't know for sure it won't.
Think about the research and testiong before any new vehicle goes on sale. Okay, maybe GM doesn't do much (my sister has an Ion with the ignition switch) but most companies spend more time testing and design than actually building an individual unit.