I don't have any ideas on how to prevent cord theft but do have a way of perhaps minimizing the cost of replacing the cord.
I wanted the convenience of a detachable cord but didn't want to spend the money for a marinco setup. Found a 30 amp 110 v recessed twist lock inlet plug manufactured by Leviton comparable to the 30 amp plug sold by Marinco, but without the water proof cover. (Cost about 25 dollars as I recall) I mounted a handy box for the Leviton plug inside the service bay of my 67
Caravel, mounted the plug in the handy box using a standard 4x4 cover plate with a single round opening and wired the twist lock inlet plug into the umbilical circuit.
The box and inlet plug are now inside the trailer and out of the weather so no water proof inlet cover or cord plug needed. Bought a 25 foot 30 amp RV extension cord at wall mart. Cut of the plug on the trailer end and attached a 30 amp non-water proof twist lock plug to mate with the twist lock inlet plug in the trailer. I plug the cord into the outlet in the service bay and run the cord to the outside of the trailer as though it was the permanently attached OEM cord. Now if someone wants the copper, they have to cut the cord outside the trailer not simply unplug it. It is relatively cheap to replace the cord by simply buying another extension cord from wal mart and salvaging the trailer end twist plug from inside the trailer from the remaining piece of the cord that was cut and reusing the plug on the replacement cord.