The Brake-Lite Relay Kit from Roadmaster is a must if you are installing a supplementary braking system. With a typical braking system, the brake pedal of the towed vehicle is depressed when the towing vehicle brakes, sending a signal to activate the brake lights - even if the engine isn't running. But if the motorhome is turning and braking at the same time, the towed vehicle's brake lights can't flash because the towed vehicle's brake signal is overriding the turn signal from the motorhome.
So when the Brake-Lite Relay is installed in line with the towed vehicle's brake lights. It only allows the towed vehicle's current to flow to the brake lights if the vehicle is actually running. So the brake lights can't override the motorhome's turn signals while it's being towed.