I always use RTV Black when I install screws into sheet metal. Just a little on the threads. It's like baddbob said, ya gotta seal it somehow. The RTV is no problemo to get apart later on, so it works for me.
I really don't know what's best to use; maybe that strip caulk for installing some windows and mirrors, etc, would be best. That stuff is still fresh years later when you take her apart, and still reusable. I use the 3M strip caulk. It "never" dries out; some of the OEM caulk was still serviceable after 20 years, and some of it had hardened into like a dry foam. I guess service life in my climate is 14-16 years.