At night you could put a cable lock through your wheels if they are spoked or take a tire off if you can't cable one. I'd also add a cable lock from the trailer to the hitch (where you hook the chains) or lock the chains. A sledge can take out your hitch lock.
Cables that can be cut with bolt cutter might not be the best but the more the better.
Make sure your hitch ball mount is locked not pinned into the hitch.
You could also add a couple hidden pin alarms. They are cheap and scream when separated. Just don't forget you have them! http://www.alimed.com/alimed-basic-pull-pin-alarm.html
Anything to make a thief take longer.