A good lock that covers the entire hitch/coupler is a good starting point. The cheap ones that have a ball and bar over the top are made of zinc and can be broken with a hammer or picked easily.
I would personally recommend one of these with a Medeco puck lock instead of the included lock. Yes, it will cost a couple hundred bucks, but your trailer cost tens of thousands so it is a relatively small cost in comparison.
https://www.youtube.com/watch?v=xVtHiX12TVw
Just using a normal padlock on top without something to cover the bottom hole for the ball leaves room for a modified pintle hook to hook up to.