Small Class C - Mercedes Diesel vs. Chevy Gasoline?

You won't find a walk around queen bed in the new MB based Views/Navions. The prior years had a 24G model that had a rear slide out that had a "semi-walkaround" queen bed. Search for Winnebago View/Navion 24G - there might be a "gently used" one out there somewhere. Don't fear the Sprinter. It's a solid chassis/drive train option, if you can get over the diesel thing.
The only GMC options will be the so called "Super Cs" and they're diesels, too. Much larger/longer than your preferred length.