Locks keep marginally honest people honest. A pro will have any lock off in seconds, if he really wants your FW or hitch. Just make d@mn-sure you have good insurance and don't leave valuables in your FW. If you =must= leave such behind frequently, then check out the various GPS tracking systems.
HEY! There's an idea. Have an OnStar-like device tied to your trailer brakes. Trailer gets stolen? Use a phone app, and lock 'em up. :B Yeah, yeah, brake wires could be cut, but it would mean time that a thief might not want to spend.
Lyle