I use a hardened standard lock to secure mine to the TV. If I need to drive somewhere, as I had to at DinoValley SP (the only place where my phone would work was at the park headquarters), I snapped on my Reese coupler lock.

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Neither lock would stop a determined thief, but I was confident that since I was parked where my "if stolen" trailer would have to be towed past other South Central folks (most of whom had seen my trailer by then), certainly one would raise an alarm. If I camped totally alone, there would be none to notice! It makes me want to install an alarm that would send a message though my smartphone (again, if it would receive a signal!); I had a remote-signaling alarm on my '86 S-10 years ago. It would let me know though a pager if anyone tampered with my truck. All but one signals were false, but one time it was a theft attempt. I arrived on the scene in time to "discourage" the would-be thief. No police were summoned, nor needed. I was much younger, then.... I'm sorta surprised that nobody on the forum has installed (or advertised its' presence) such a system on their trailer, since many are high-dollar outfits, and many are very high-tech as well (solar, GPS installs, TV/satellite dish, you name it). The 12vdc battery on the trailer would power any auto security system as well.