Drill a hole down thru the two nuts and bolts that hold the coupler the tongue. Drive a hardened roll pin into the hole and spot weld it in place. Nobody, including yourself, will be able to simply take the bolts out. They could grind the bolt head or the nut completely off, but that would slow them down considerably with the chance they'll see it in the same light as Sherry and, "go on to an easier target".
Don't forget that if they come with a ball mount that has a 1-7/8" ball attached, they can simply lower the trailer onto the ball, wrap and secure a short length of chain around it and slowly drive away. So this brings up two more deterrents that are needed.
1 - You need to have disabled the power to your front jack (although they could have brought a floor jack and the tool to manually lower the jack just in case.)
2 - You need a spare, appropriately sized ball with the shank cut off to keep locked inside the Bulldog Hitch to further thwart the miscreants.
Or, you could just bury a perimeter of M18A1 Claymore's around the trailer. But this would probably involve a cleaner and/or a lot of paperwork.
Really, just buy a Proven Lock for your hitch and fuggedaboutit.