I had a similar issue on my previous Surveyor trailer. The taillight/brake light lenses did not fit flat against the light housing. Maybe they warped a little over time. In any case when it rained, water would get in the gap and blow the hot bulbs. I countered with a strip of white electrical tape, which cured the problem. I figured the tape would be easier to remove than silicone if I ever needed to replace a bulb, but I never needed to.
I've noticed on my new Surveyor that there is a little ledge above the lens that helps keep water from getting in, even if the lens warps a little bit.
2013 Surveyor SP220
2010 Silverado LT 5.3 V8
The world is a great book, of which those who never stir from home
read only a page. - St. Augustine