I don't think much can stop a determined thief; however, with that said... the time it would take the thief to overcome the locks and or theft prevention methods used is also a factor...
-- There are some serious hitch/coupler locks out there; the better ones are increasingly more difficult to overcome ...
-- Wheel locks also help ...
-- Lights are also a great deterrent ...
-- Noise is also a great deterrent, e.g., alarm systems ... Noise and lights are great together ...
-- Security camera/s might help catch the thief and even get your rig back ...
-- Parking it out of site, out of mind, e.g., back yard, shed, garage, etc.;
AND a layering of these is even better ...
Remember, the lock usually just keeps the honest guy honest ... safes can be picked, but place it in some inconspicuous place adds another level of security...
BTW, most folks I know with TD's, park them in a garage, shed, etc. for trailer protection and security...
Anyway, you get the idea ... I hope this helps you come up with an idea/s ... Remember security is best kept to yourself, people talk and it doesn't take long for others to know the basics of your security methods ...
