Well, I can only speak from my own experience- I guess that makes me a little
bit of an expert!
My motto seems to be "If a thing is worth doing, it's worth doing TWICE !"- I usually get it right the second
My experience with patching leaks
without removing the vent/window has been that I'll have to do so eventually anyway... patching with silicones, etc. hasn't worked very well for me.
It's easy enough to remove a vent cover- I think. As I said, I'm not familiar with Casitas- if the cover's plastic and brittled by age/sun there might be a risk of breaking it while removing/replacing.
If it's riveted on, you just drill out the rivets, pry off the cover, clean the old caulk off, put the tape on , and re-rivet the cover back on the trailer.
I wouldn't go to all that trouble without making sure
that the vent is the leak source...I've run a garden hose over various spots to accomplish this.