This may or may not apply to your situation. I have a 2020 Chevrolet Spark that I tow with a 2018 Mercedes based class C that also requires the car's negative battery be disconnected. I also use a Blue OX baseplate and tow bar as well as a Brake Buddy supplemental braking system. In my towing set up, I have also included a 12V charge line to the car battery powered by the RV.
The charge line to the car battery created a problem in that the metal to metal physical attachment of the tow bar to both the RV chassis (through the hitch) and the car chassis (through the baseplate) supplied the RV negative ground to the car. If I just disconnected the car's negative battery cable, I would still have a complete 12V circuit powering the car's systems provided by the 12V RV positive battery charge cable to car battery and the negative from the RV chassis ground to the car's chassis ground. The RV would be powering the car's systems.
I solved this problem by disconnecting the positive battery cable when towing. disclaimer: One has to be very careful when disconnecting the positive cable, with the negative cable connected, so that you do not touch any grounded metal with your tools. You can first disconnect the negative battery cable before disconnecting the positive cable and then reattach the negative cable. I am only telling you how I solved my issue and I am not recommending for anyone to do what I did. Emulating these procedures would be at your own risk. Disconnecting the positive battery cable can result in both damage to the vehicles and injury.
Unfortunately, the positive battery terminal on the Spark also has an electrical device attached. The cars positive runs through this device but the device can be detached from the battery clamp consequently cutting off battery power to the car's systems. The first photo shows the device. The same photo also shows the positive connection for the charge line from the RV and the positive power connection for the Brake Buddy connected to the positive battery clamp. The 2nd photo shows the positive device secured away from the battery by tying it to the washer fluid filler tube.
The 3rd photo shows a factory ground screw that I used for the negative of the Brake Buddy, the RV to car wiring harness and charge, and the Blue OX tail light bulbs in the rear light housings.
The 4th photo shows the Spark's battery configuration before I attached the power cables for the Brake Buddy and the charge line. A single nut disconnects the device from the positive battery clamp.
If you do not have this same positive device in your system, it would be a lot easier to put on a positive disconnect. I have seen photos using a blade switch on the positive battery terminal. Again, you have to be very careful and, as stated above, proceed at your own risk when working on the positive battery cable with the negative battery cable connected.