Most deterrents attempt to keep the trailer from being taken from its parking spot. Unfortunately, as shown in previous posts, thieves observe the trailer and its setting pretty well and figure out the best time to take it. They also realize how to o it fairly quietly and unobtrusively.
The GPS trackers probably work pretty well as long as they have a self-contained power supply (battery) and are hidden well enough (not in the recommended spots in the tracking system owner's manual that the thieves might also have read!). If GPS trackers become popular enough, the thieves will probably start doing what they do with cars (especially Onstar type vehicles). They steal them and then park them in a hidden "staging" area for a few days to see if the police show up.
What also might work is a system that gives the thieves a headache AFTER they have taken the trailer and are on their way. An alarm system going off (again, separate hidden battery) and locking up the trailer brakes on a public route is not the best scenario for thieves. If they can't defeat it fast, they'll probably just unhook and run with their TV. At that point the police would be investigating an "abandoned trailer" on a public road and hopefully contact you (via your plate #).
Steve