You could also just make up a cable for the brake buddy that attaches to the car's battery. Disconnect negative cable from the car, add a negative clamp that's wired to the brake buddy. You'd want to stop traveling and start the car every 3 hours to charge it up.
You could also just replace all the 'required' towing incandescent light bulbs with LEDs. (And get a modified flasher). That would lower the current supplied by the RV so you would have enough to run the brake buddy by stealing from the 'Running', 'Brake' and 'Turn Signal' wires. Which is what that product mentioned earlier does, I believe.
What I did with my setup is add an appropriately sized wire to one of the unused pins on the 7-Way connector so I'd have 12 Volt back at the towed through the tow electrical cable, and the 6-way at the car. I used that to charge the battery, but it could also be wired directly to the brake buddy.