In my experience the interface between the lens and white plastic housing does not come with a gasket. You might try making a gasket using plumber's putty or a strand of rope putty. Should work and any excess that squeezes out should be easy to clean up.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.