It is always something! In Tennessee, you don't have to register a trailer at all, BUT, if you decide, you can apply for a one time 'Semi trailer' tag and pay a one time fee, and never have to reregister the trailer again. Contrary, if you plan on crossing State lines, you have to have the trailer tagged.
The VIN does come up in the DMV data base, because we have had folks here that have purchased one ... to get a 2008 Trailer Registration, and in other states, the VIN registers as a 2002 Trailer.
One of the guys that purchased one that lives in North Carolina, sent off for his title and it took almost 8 months, and then he got a letter stating that a title officer (North Carolina Highway Patrol) wanted to inspect the trailer. They came, they looked, and they issued him a 2008 Trailer Title and Registration.
This State verses Federal Law thing seems like it needs to be addressed ... on a nationwide bases.
Dendy