The model numbers reflects nothing. It's just a model number.
Jayco and Starcraft have clones with different model numbers. Most makers do not measure the box at all, they measure total length.
Rockwood's model numbers are close to their trailers total length.
Go by the makers specs to find length. If they don't offer a box length, as most, then they are not measuring it.
If you want to find your box size, you'll have to measure it yourself.
Campgrounds use total length when making reservations. You are not going to take the box off the frame, so the frame has to fit into the site as well. ;)