Hi,
I 100% agree that when towing you want/need to have the coupler pinned or locked closed.
That said, I pin mine.
I do have a long shank Master Pad lock in the camper that I have 3 of them keyed alike. I use 2 of them on my generator cable and the other on the bicycles.
Here is a quick hit on a similar one. Mine came in a set of 3.
Master Long Shank lockI use those 3 locks for multiple things on the camper from time to time.
As far as camper theft, just locking the coupler will not slow down a low life from stealing your camper. If you have a 2 5/16" coupler like many campers do, they can steal the camper with a 2" or 1 7/8" ball and haul it off to later cut off the lock and use a 2 5/16".
For security, I use this one. It works with that Shelby coupler
Master Ball Coupler LockThe ball coupler types of locks slow the low life down, but if they want the camper bad enough, even these will not stop them. If there are a lot of campers around to pick from, using a ball coupler lock may deter them to move on to easier picking.
If you have an old tow ball, cut the thread shank off of it, put it in the coupler and lock the coupler. It works as good as the store bought ones.
Hope this helps and good luck
John