If you have to leave it there, get one of those little 12V electric motorized trailer moving dollies. Then turn it around 180 degrees and put the hitch end up against something like a house, garage, tree or whatever so nobody can back up to it.
Then get one or two of those wheel immobilizer locks and secure the most visible wheel.
Then get a high end hitch/tongue lock. And for good measure, some stout high grade chain, around an axle to a tree or light pole. Or the hitch to some immovable object.
A motion sensor with some floodlights trained on the area will help too.
When a thief sees all that, they will move on to the next target.
