Roadtrek Tail light doesn't work

You could wait until your dash lights go out and then you will know you have no tail lights at all. Trust me it's the ground wire. Do you have a wire tester (PHOTO #1) Clap to the vehicle's metal body and stick point into the wire. (not hand or fingers) If you have the Bulb in and lights on, doing this should turn the light on unless the ground is bad. If you had any work done say to your trailer tow hitch with trailer lights that may be the issue. Trailer lights may work but, one of the vehicle lights goes out. Re-due and check wiring.  I think the Bulb is a "1040" Which lights up when stopping, Running lights are on, or blinkers are on. Let's us know how it works out for you.  Good Luck and safe Travels.