I have a Klampit and it is only really a visual deterant. Locks only keep the honest out, and if they want it, they will take it. These days with cordless tools, I could remove my Klampit with a grinder in under 30 seconds.
Since you have a treg hitch, its a little harder for the person to steal it that is doing so on impulse, however, it is easily organised to get the right hitch for the car in preparation.
Tow ball cop is a good option for the standard hitches but unfortunately this is not an option for you.
I work on two theorys for mine. Give them heaps of stuff to do to take it. Eg keep it stored with the legs down, put a cover on it, install the Klampit and put a chain between the spokes of two of the wheels joining them together. None of this will stop them taking it but it will slow them down and will make noise and hopefully get them noticed.
A chain from the axle to a plate bolted to the ground is also an option but I have not done this as knowing my luck, I will forget to undo it and end up with my axle ripped out from under the van!!
My other theory is put a tracker in it. At least that way if it goes, you stand a chance of getting it back. My tracker cost $150 by memory from Jaycar and I use a Telstra telemetry sim which cost $5 per month plus each text message it sends.