We had the same symptoms with all the lights (3) in our slide. Fuse and bulbs were good and I also checked the voltage at each light, all over 12 volts. When any light was turned on, the voltage would drop to 0 volts. I don't remember why but at one point, I removed the positive wire from the fuse block for these lights and touched it to a known good positive terminal. Voila, the lights came on. I then re-attached it to a spare terminal on the fuse block, utilizing the original fuse. All has been ok since then. I guess it was a bad circuit on the converter.
Good luck with your search, I finally figured it out after 2 years of fiddling around and a month stay at the dealer.
🇨🇦 Scott & Janie 🇨🇦
2004 Dodge RAM 2500 CTD (slightly tweaked)
2011 Cruiser 31RK
Cooper the Golden Retriever, and
Sam the black lab.