Anything that starts as an incomplete, cut-away van cab and bare rear chassis vehicle and is finished by adding a motorhome house body onto the chassis and attached to the cut-away cab, is considered a class C, so far as I know.
A class B is a complete, one piece factory van body vehicle, which the interior is filled with RV living accessories.
A class B+ is a class B van body vehicle with RV features inside, that has a raised roof added on top of it, so far as I'm aware.
The Sprinter van RV's I've seen appear to be class B or B+. I haven't seen any class C Sprinters so far. That doesn't mean they don't exist though.