Reverse lights only work with parking/headlights on

Re-Reading, Some of the discussion involves Trailer Wiring. Used to be, Trailer was just wired into Chassis, but Ford has had supplemental trailer wiring for years. Anything's possible, but I doubt the Tioga coach body was wired as a Trailer.
What did the re-wiring you did involve? What were you trying to create or solve? It's a good troubleshooting precept to suspect recent work and modifications.