I have 2 suggestions. First is to take a hitch ball and cut the threads off of it and insert it into the hitch and lock it with the best lock you can find. Second works the absolute best but is more difficult. Years ago a friend had his boat stolen from his carport in the middle of the day. He cut a 2'X2' hole in the concrete under the axle section of the trailer of his new boat. He dug that 3' deep and put a piece of 1" rebar in a loop just high enough to get a cable through.
Fact of the matter is that if they want it they will come prepared and get it, but these are a couple of options. A swing away tongue and remove the hinged portion is another.
Creativity is just intelligence fooling around