Physically the length of the TT is bumper to ball socket. Functionally the length of the TT is the length of the box. Practically it make no difference which you choose to use.
If you are storing, hauling or going to enclose the TT, then the physical length would be used.
Campgrounds do not seem to have any set rules. They accept what you say. Typically 25 ft., 30 ft., 35 ft. and etc. is detailed enough.
The manufacturer's Model numbers seem to bear little resemblance to the real world dimensions. They are just the maker's convenient numerical descriptions.