I can't watch the video now, so sorry if I'm repeating, but you'll need to remove the vent from the roof in order to re-seal it if it is leaking. After you remove it you'll see that it has a flange around the outside. You take butyl tape and place it on the flange (cover the whole flange - the part that will rest on the trailer roof), then reinstall the vent and re-fasten it to the roof.
I would speak strongly against using any silicone for sealant. In my opinion it does not do a very good job, and it is the devil to remove (and even if/when you do manage to get it off, the contaminating residue has about a 300 year half-life
). If you use open-ended rivets to install it (you can also use sealed end rivets or machine screws with nuts) then you would want to put a dab of caulk over the rivet's hole, but I would suggest something like polyurethane or perhaps polysulfide, not silicone. Other fasteners take sealant under them, not on top.
I also am of the opinion that you don't need any bead of caulk around the outside of the flange if you've used butyl (or caulk) under the flange. That's where it really counts.
Raya
(sorry if I seem to opinionated; of course you don't have to take my recommendations either
)
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