What do you use as the "length of your travel trailer"?

It always appeared to me that the model number is the length of the "box," and the length of the trailer from ball to back bumper is about 4 feet longer, give or take. And when asked the length of my 17QB TT, which is 21 feet long, I always say it's 21 feet. As others have said, I want to be able to actually get my TT into my assigned site. And sometimes when they say "fits 21 feet" they really mean 21 feet and not an inch more.