Model numbers generally indicate box length with obvious exceptions. As someone else said, add about 3.5 for the front frame. My 24 Jag was about 28' total and my 28' Bullet is 31.5' long. I had to measure it closely because I didn't have any room to spare on the side of the house.
No CG has every indicated what specific type of length they asked for. For my 28' TT I usually just say 30'. But I also try and use Google maps to see not only how long the site is, but how wide. It is sometimes an issue finding space to park the TV if the site is smaller.