Same thing on a previous Surveyor I had. The lens became a little warped and let water in at the top, blowing out the hot bulb when it rained. I put a strip of white electrical tape across the top. Didn't look too bad, and it was easy to get off if I ever needed to replace the bulb -- which I did not after applying the tape.
The new Surveyor has the same tail lights but it has a piece along the top of it that seems to be designed to keep water out if the lens bows. Same problem, different solution.
2018 Coachmen Apex 249 RBS
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