Even in older tow vehicles, if the "hot" wire was installed properly, it should have a relay in the circuit that is tied to the ignition switch. When the tow vehicle ignition is turned off, the "hot" line in the umbilical should be dead.
If the hot wire circuit did not include a relay, it can be easily added. Usually, an old Ford starter relay is wired inline between the tow vehicle battery and the umbilical connector. And, this is activated by a wire that runs to some connection that is hot when the vehicle ignition is on. It's probably easiest to have a tech at someplace that installs hitches do this for you (e.g., U-Haul). However, the actual installation is very simple.