[Fulltiming]

Those lights are notorious for dimming and flickering or just going out especially if the voltage is even a little low. The adapter cable is designed to direct the 120 volts to both 120 volt legs in the coach or else 1/2 of the circuits would be dead. However, you are still limited to the max current of the circuit you are plugged into.

Mine are labeled L1 and L2, if your are labeled R120 and L120, I would presume that would mean Left and Right.