My next door neighbor is a remodeling contractor, and he has a couple of large covered trailers that he hauls his stuff in to job sites. He takes a length of steel pipe that is about an inch longer than the distance between his trailer tire wheels. It has a steel ball welded to one end, and a hole drilled in the other to run a heavy lock through. Whenever the trailer is parked, even in front of his house, he runs the pipe through the wheels and locks it. He also puts one of those plastic trailer ball locks on the tongue. He has a similar lockup that goes across the doors in the back. Any of it can be defeated by a thief given some time, but at least it serves as enough of a deterrent to hopefully make them go looking elsewhere for easier pickings. He even armored the inside of his trailer walls, ceilings, and doors with welded-in perforated steel panels.
Of course, this was an open trailer that got stolen, but it never hurts to make it more difficult for the thieves. If they had been unable to hook up the trailer and haul it away, they might not have been able to steel everything as easily. Generally, anything that can slow a thief down helps since they don't usually want to spend a lot of time where they might get noticed.