You can usually find small padlocks that will fit in the pins. That's what I use on our car tow bar, which has similar pins. And they make special locks for tow bar pins - maybe they would fit your hitch?
When I park the 5W, my kingpin tripod has a place to put a padlock. It seems loose, but the shakle is large enough that the tripod cannot be removed (well it could if you held the cup and spun the tripod, but still would not get the cup off the pin.
I think the hitch in the truck is what you are trying to secure? I have a lock set from Reese that are slotted metal plates that slide over the end of a standard type hitch pin and then accepts a med size paddle lock. You can put this on 3-4 spots on the hitch so no parts can be removed, without cutting the locks. Should slow em down.
I put a rolling lock cover on my truck just for this reason. When traveling cover is open, when we are camping the cover is closed and locked so we can go see the sites without any worries. The other plus is when we stop at an antique shop the wife is limited on buying large items.LOL ;D