Brush wheel on my drill and acetone.
Scrape as much as you can with some scraper, putty knife, flat blade screwdriver, etc, whatever you have.
Then run the brush wheel. It will not come all off.
Clean the window flange with a old rag and acetone. A thin film of silicone should wash off, thicker part will soften up.
Run the brush wheel again, then the acetone rag again.
In my case, after the 3rd pass, I was down on bare aluminium pretty much everywhere.
Took just a few minutes.
I found it harder to remove the old silicone off the fiberglass. I used a plastic spudger as a scraper to remove as much as I could without scratching the gel coat, then i applied some Dab's Silicone-B-Gone remover. Let it work an hour or two (cleaned the window as above in the meantime). The silicone remover softens the old sealant and turns it into some sort of gelly that can be scraped off much easily or even just wiped with a rag (2-3 applications were sometimes required).
I've done all 7 windows on my trailer, lots of pictures on my thread: