I would never use Silicone Caulk because it does not last.
UV just wrecks it and it is hard to get off without scratching.
If what you are doing is "permanent" use a good quality marine cement.
A "Warning" though, every thing I mounted that I thought would be "permanent" has either broken and then needed to be replaced or I got tired of it and changed my mind.
Eternabond makes a double sided sticky tape for mounting stuff to the exterior that comes up with a hair drier or heat gun.
EternaBond Doublestick MicroSealant Putty Tape
If it needs to come out of a tube and water intrusion is your issue, Dicor can't be beat.
3M Marine Cement is the way to go for seals that come loose.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW