I use a locking pin for mine, too. And a suggestion I got at Americade was that if it was a 1 & 7.8" coupler, lock a golf ball in it so no one can just put a pin up in it and tow it away that way.
Here's a trailer that was parked at the same motel as I was for Americade - I think this guy may have gone a little overboard, but it was NY plates. Maybe he lives in the City.
2003 GL1800 MotorTrike - "Canth"
Past Master, Franklin Lodge AF&AM Grafton MA
Member Libanus Lodge #47, Bristol VT
Widows Sons VT - Charter Member
Lugeless Pavement Racers - Member #6
NRA Certified Instructor
FFL - Lonely Mountain Arms, Starksboro, VT