As to whether a 20' would be a class C or a B+, it's somewhat of a moot point in as much as "B+" is really just a marketing term and doesn't have a very solid or uniform definition. A B+ is a class C that usually is on the smaller side of things and usually has no cabover bunk. I guess B+ sounded better than C- to the sales office.
The classes are based on what sort of a chassis the RV maker starts out with. A class B starts out as a cargo van, and the exterior walls are the van walls. A class C starts out as a cutaway chassis (or possibly a cab chassis), and all the exterior except for the driving compartment is added by the RV maker. A class A starts out with a bare chassis and all the bodywork is added.
A bus conversion is usually considered a class A if anything, though it seems to me that logically it ought to instead be described as a really large class B.