I've had so many leaks to repair that I tried many types. No leaks now after using everything from silicone to the poly's. One place in particular I used only rv type byutle (sp.) type tape/rope caulking without any sealant around the outside.
I discovered actual Vulkem lasts a long time. Discovered removing the part, cleaning and caulking between mating surfaces is key since the sun/ weather cannot get under there.
Also discovered byutle tape is made in varying degrees of quality and the industrial grade from heating and air conditioning parts supply houses are the stickiest/ best to use. Hardware store, rv store quality is usually just non professional grade by comparison.
Last, the byutle tape is loads easier to clean off and redo. Because of this I have left off the perimeter caulk that is so hard to clean off for redoing it later.