We use a Husky kingpin lock. I can't find the link, but it is a 1/2" steel collar that fits around the kingpin and has a 5/8" pin that goes through the back with lock similar to a receiver hitch locking pin. We also disconnect the battery so they can't raise or lower the jacks (can still be done, but will slow them down some). Also am impressed that Heartland uses a different storage door key than the "universal" one that most others use.