It's not the exterior you need to seal but the hole behind the light for the power wires. Completely remove the light and use plumbers putty or similar material to create a seal the wires pass thru. Also goop on sealant such as caulk to the threads of the hardware that hold the light in place.
Mark, Jean, Paul & Lizzy (the mutt, RIP) 1997 Fleetwood Southwind Storm 34LS > Thirsty, noisy & clunky. She ain't pretty, but she sure is fun! "Aerodynamics are for people who can't build engines." Enzo Ferrari
I use plumbers putty. Since I know I will have to open them to change the bulbs I wanted something that would seal well but be easily removed. In the first few months of owning our latest coach I must have changed 3. In the last year I haven't changed any.
Clear urethane works the best. I also changed out all of my older incandescent clearance lights for SEALED LED clearance lights so that I did not have to worry about water leaking around the removeable lens.