Unfortunately, at least in my state, its very easy to steal trailers. You can re-register any trailer as a "homebuilt" trailer. In fact, I had to do this with both my first pop-up camper and my boat trailer. Neither had titles when I bought them. You take them and get them weighed, then take the weigh ticket to the BMV and they will make you a new registration. Don't even have to get a title for it, you just need a registration.
My mom's boyfriend had a 20' trailer stolen from where he bought wood, they would load the trailers as they had wood, then call when its ready. He showed up to get his trailer and it was gone.
The best thing you can do is make yourself less of a target. If it takes 5 minutes to steal your trailer and 2 minutes to steal your neighbors, they'll take the other one. I add a ball lock, put the stabilizers down, and run a chain through the wheel rims.
At home I have a zoneminder IP Camera system setup, and one of my cameras is aimed at my backyard and records any motion. This includes any cars driving down my side street, etc. Its a 1080P camera and captures enough detail to make out license plates.