I don't think anything at all can be derived from the numeric portion of a floorplan. With that understanding, hopefully this will alleviate confusion in trying to derive lengths etc from it.
How long is a Landmark Key Largo? A Landmark San Antonio? How about a Cyclone 4100? The coming 4200?
Just some numbers and names. Always, always go by the lengths listed in the literature. King Pin on a "standard" pinbox to rear cap (outside measure) is how 5th wheels are measure. So if you have an extended pinbox or a rear ladder, those extensions would be added by you to the published length.
How a Travel Trailer is measured, I am not sure. My guess is the Ball Coupler to the Rear Wall (outside measure). But I could be wrong on this.