I may be considered still a newbie since we have owned our first TT less than a year, but I would also say the number is generally the living space box.
In all of my pre-purchase research (the wife would say I spent many months online reading specs and looking at floor plans), I don't ever remember noticing a unit that was given a model number related to overall length instead of living space box length.
I also figured out most on-line reservations for campsites, so far, have asked for overall length of camping unit.