It's all over the board with no rhyme nor reason.
We just bought a 22RR...the box is 25ft and the overall length is 28ft.
If you look in the brouchers, typically they do provide an overall length figure, which does seem to be hitch to rear bumper pretty consistently.
Of course, in most cases the exact length isn't critical. If you compare an 18ft to a 32ft, it's a big difference but it will also be obvious. If you compare a 26ft to a 28ft, you are probably better off seeing them in person to get a feel. If the 26ft is well laid out, it may be more comfortable than the 28ft.