From past experience I've found that most lighting problems on these motorhomes are usually caused by bad ground connections. I would start your troubleshooting efforts by tracing every ground wire and ensuring the connections are clean and solid.
I spent half a day one time with similar issues that all ended up being cause by one ground connection.
Good luck and let us know what you find.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)