Hitch Lock

Most of the time, you really don't need to worry about your trailer getting stolen.  But...there ARE times and places where the theft of your trailer is a distinct possibility.  Buying and using a hitch lock is cheap peace of mind for those times.  There are Many variations of hitch locks and one can spend a LOT of money on such a device.  Bear in mind that a hitch lock only deters casual thieves.  A determine thief can cut off ANY known hitch lock. Typically, however, such skilled thieves have bigger fish to fry and won't be interested in your small one-axle travel trailer.

http://www.harborfreight.com/ez-lock-trailer-lock-98518.html