When I replaced my marker lights, I didn't fuss with the incandescent bulb. I just removed the bezel, drilled out the two rivets and removed the whole assembly. Some of my bulbs were corroded to the fixtures. If I had to remove them, I would have broken the lamp glass and pulled them with needle nose pliers... but why? If the $10 LED marker lights are too salty, you can get new incandescent marker lights at Wal-Mart for something like $2 a pop. Trust me... getting a bulb in and out of a new fixture is much easier than prying them out of the old ones.
As for working on the trailer, if you don't like being on a ladder, I'd suggest some movable scaffolding
. I don't think anyone should be laying on the roof of an Airstream... whether from a safety standpoint or a damage-to-the-trailer standpoint.
Oh, and I used vulkem to seal the lights rather than silicon.