There have been a number of folks on the T&TTT forum that have had their teardrops stolen and one cargo conversion. Being a bit obsessive about the whole thing I decided to really look into it and the answer is, if some one wants it badly enough they will get it given enough resources and time.
If you can get a bolt cutter on it it is toast and this includes the vast majority of trailer locks, Do a Youtube search on bicycle lock testing and trailer lock testing and see what I mean. I changed the trailer hitch to a bulldog hitch so I could use one of these from Gus Hill http://66.147.244.244/~trailet4/shop/ He and I had a long chat concerning design and materials used.
BUT as I said if they want it badly enough it is gone. You can use a GPS tracker which can be hidden and powered from the 12V system but ultimately there is no substituent for insurance, though custom graphics and paint can help.