OP asks "Why not get rid of the B and C classifications and just go to Large, Mid-size, and Small/compact Motorhome designation as the car industry uses? Or are the B and C designators just a marketing tactic?"
Not positive, but I don't think you have to go too many decades back to a time when there were motorhomes (slang, 'Winnebago'), mini-motorhomes (chassis cabs with coach built on back), and camper vans. I'd bet few rigs in each MH category strayed far from group average lengths of 24', 20', and 16' respectfully.