Tim,
Sorry to hear about your theft. It's too late for your stolen trailer, but on mine, I use one of those ball hitch locks, whether it's hitched to my truck, or sitting in my yard like yours does.
That lock may not stop a determined thief, but it will surely slow down whoever wants to steal it, as they have to hacksaw it off before they can raise the ball hitch lever. Without the lock, they can be backed up, and hooked up, and gone, in just a couple of minutes.
I think they're less than 20 bucks.
I also use a lock on my hitch receiver. We were travelling with friends in their motor home several years back. He was towing his car behind the motor home. We stopped at a restaurant for dinner, and we both checked all the hitch stuff, and the wiring harness, before going into the restaurant.
Came out after dinner, pulled onto the Interstate, to suddenly find out the car trailer was loose, and diving left and right by the safety chains. Stopped without further incident, and found someone had removed the reciever pin while we were eating dinner! I was able to find a bolt that got us off the highway, and we stopped and bought a locking one after that. Ever since then, I've locked both parts of my trailering system. Even if they don't intend to steal it, they can still hurt you financially by pulling stupid stunts like they did to our friends. $40 is good insurance for piece of mind.