It depends. A Winnebago ERA and a Winnebago Trend are the same length. The Trend is fatter and taller by about a foot. However, the Trend's original chassis came from Mexico as a cab cutaway, and the ERA came out of the Mercedes factory as a van.
This is why an Excel and a 210 are considered a "B" motorhome, even though the rear is a fiberglass shell.
AFIAK, that is the difference between a "B" and a "C". If it came to the RV maker as a van proper, even if the entire walls are removed, it is still a "B". A cab chassis? A "C". Just a steering wheel on a ladder frame (requiring a custom facing by the RV upfitter), an "A".
Does it really matter? It may with resale value. If one cared about rig usage, this would depend on the individual model. With the unibody construction of modern vans like the Sprinter, Transit, and the Sprinter, there are a lot more engineering/space issues to deal with, as opposed to just grafting a box onto a cab cutaway.