I had two issues with my campers marker lights. One was that the light had been installed "upside-down." That is the weep hole was on top. So when water seeped in between the base of the light and the lense, it tended to build up and not drain as it should. Check for this by snapping off the lense and look for a little notch in the base that should be located on the bottom.
The second issue apparently was caused by an installer who drilled the wire access hole for the light slightly off center. When he attached the light it left a very small half Moon hole exposed at the bottom of the light. The hole was such a small sliver that it easily went unnoticed.
The fix was to turn the light around and caulk the small half moon hole.