I'm late it reading this thread, but I have what I feel might be usefull information. One of the 5ers we owned had a water leak I could never locate. I spent many hours searching, caulking, and tightening. One time I was working near a clearance light and noticed it had water inside the lens. I removed the lens and naturally the water ran out. Then I noticed the water high level mark on the base was even with the bulb, behind the bulb was the wiring hole. I removed the base and discovered water had been collecting in the lens until it reached the wiring hole, ran through the hole into the hole in the front cap, and the rest everyone knows.
My solution? I drilled a 1/8" hole in the bottom of every lens on the exterior of the 5er. Never had another leak, but it took me a full week to repair the water damage so we could sell the 5er.
I hope this is useful in some way.
P.S. The name of the RV leak test system is: Sealtech Manufacturing Inc.
I insisted the dealer use this on our last 5er at purchase or we wouldn't sign the papers.