Silicone sealers break down their ability to bond, when exposed to the sun.
Therefore, any silicone sealer should "NEVER" be used on the exterior.
Airstream learned that lesson in the early 70's.
For those that don't know, when the silicone sealer lets go, the water leak appears again. Now the real fun begins. In order to use the correct material, either Vulkem, Parbond or Sikaflex, the surface "MUST" be chemically clean.
When you applied the silicone, you created a barrier, that must be removed chemically, completely. If not, none of the other sealers will bond.
Lastly, installing silicone sealer on the roof makes it difficult to inspect. Therefore you will find when it has let loose, after a good rain. Problem" Too late!!
Lastly, and for those that like good looks, attached is a photo of what silicone will look like in a couple of months.
Not very eye appealing. Not good for the trailer. Not good for the owners pocket book, to fix the problem.
Not much left to debate.