Here's what I would do, for max effect at min cost:
As suggested above, get a large, screw-into-the-ground anchor, like the kind used for mobile home anchors, in the biggest size you can find. Screw it into the ground near the front of the trailer, with the top angled away from the street. That will make it extremely difficult to pull out because the shaft will bend first.
Get a large motorcycle cable and lock, the kind that are quite resistant to cutting. The good ones come with a loss warranty, so take some photos of the finished installation just in case. A serious chain would also work and cost less, maybe...
Lock the cable through the A-frame right near the coupler, so the setup is obvious to the dumbest thief. That way they won't take off and rip the axle
or bumper off. They can't unscrew the anchor with the cable through the eye on the shaft.
After you have the new egg crate built, you can reuse the anchor to hold Fido and convince your wife you need a motorcycle now that you have a cable to keep it safe