I'd pull the lamps out of the left set of tail lights, look at them closely and check each bulb very carefully. A while back I had some strange issues with the lights and found when I checked the bulbs a filament in one of the dual filament lamps had blown and the dangling piece of tungsten had welded itself to the second filament inside the globe.If all are well there I'd start on the 7 pin connector, but that's a pretty sturdy plug and unless it was hit hard or had a hard pull I doubt it would just fail out of the blue. Good luck. These type of problems can be frustrating and time consuming.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad