I had originally set up 4-way wiring to a 7-way socket on my van then later added a +12V power line and upgraded the ground wire when I started to tow a travel trailer. I used 10 gauge wire with 30A fusing.
You might not absolutely need a +12V line if only towing a car. It depends on what car you're towing, the lighting used and the braking system added to the car.
I've towed a car without the +12V power line. I had to stop every few hours to start the car to charge the battery a bit to keep it usable. The portable braking system added to the car had a small compressor that would run and draw from the car battery. The car I'm towing requires periodic starting and running through the gears on the automatic transmission to circulate the transmission fluid.
Currently, I use a +12V charge line when towing that same car with a different RV. I still need to stop and run the engine to circulate the fluid in the transmission but don't need to be concerned with the car battery state of charge.
I just ordered a Roadmaster 690 parking light diode to use as the diode on the charge wire. Most of the lower cost toad charge wire diodes are not rated to permit much amperage to pass (some are rated less than 1 amp) so that's why I'm going the DIY diode route. The Roadmaster 690 is rated 85 amps according to e-trailer - https://www.etrailer.com/Accessories...er/RM-690.html
- that's way more than I need so I expect the part to last a long time. It is replacing the existing low amp toad charge diode. The existing fusing stays in place.