I got a master lock tongue lock. It has a ball on it, you slide the ball in, then tighten this gizmo over the coupler and key lock it down. Its the best tongue lock ive seen. Ive also got a master lock cable/lock that I tie up the wheels and lock it down to the trailor.
Anything can be stolen with the right tools. You just got to make it difficult enough to where nobody wants to deal with it.
