Hi Richie, you are a new guy with an old rig, eh? I would go around refreshing chassis grounds. Maybe start with the main one from the negative chassis battery terminal to the chassis frame, then on individual ones for the lights you're having issues with like the tail lights. I don't know the Sun Voyager, so I can't help with fuse location and the like though.
A refresh of the coach battery(s) to ground refresh might be in order too. It won't help with the marker lights, but it might help with coach electrical gremlins.
2005 22' BT Cruiser 5211, Chevrolet 3500 chassis w/6.0L Vortec V-8; 6'x12' Cargo Trailer; Honda CB500X motorcycle for street and ADV riding; Yamaha TW200 motorcycle for dual-sport and trail riding...All of it for fun and adventure!