Exterior lighting systems are provided for towed vehicles, such as towed RVs, which includes a control module for receiving conventional illumination signals from the towing vehicle through a conventional trailer plug, and supplementing those conventional illumination signals with synchronized supplemental LED lighting. That supplemental lighting can include “constant-on” flashing LEDs indicative of operator intentions in the towing vehicle, either at locations adjacent conventional exterior lighting or other locations on the trailer. The control module can also support supplemental exterior illumination of the RV when the towed vehicle is stationary and in use for camping or advertisement. The control module includes a failsafe mode in the event of a fault in the supplemental lighting (so as to maintain conventional exterior lighting capability), diagnostic LED output for troubleshooting, and an automatic reset feature.