"In other words: How much longer would a Class C be to have the same living area length as a Class A? I'm guessing it's not only the length of the "dog house", but also the length of the engine compartment."
Width, height are fairly similar in either. Our Navion has about the same distance from the windshield to the "living area" behind it, as our Sunstar 26HE had. The only difference is the Navion doesn't have a doghouse. The Sunstar did.
Also depends on the floorplan. Most C and entry level A class coaches have similar living areas, including a galley, opposite a couch and dinette, usually situated in a slide, or slides.
So the answer would be whichever one is longer, overall, will probably give you the most "living area".
I would have thought this would be obvious?