I never thought of trailer theft until it happened to a friend a month ago. She stored her toy hauler in a lot in FL (she lives in a townhouse so no place to park it) and went to pick it up only to find it gone. The detective told her that it happens way more than people think. He said the thieves remove the VIN plate and replace it with another one from a junk trailer and sell them quickly through craigs list, or just haul it to a permanent resident style campground where it sits and is never tagged. I was shocked...luckily it was a new model and her insurance covered her loss as well as content, but I can imagine on an older model the payoff would never be able to cover the replacement cost. He suggested that you mark the VIN number inside the trailer, like on the inside of a few cabinet doors....and of course anti-theft devices.
