My buddy who works as an RV tech at a dealership and does windows all the time never uses silicone. To keep jobs from coming back, he uses double butyl tape.
I tried it. I like single because the window goes farther into the slot for a tight fit. I have NEVER had one leak on me, even years, and 50,000 miles later. Double tape MIGHT be called for IF one were pressing the window into the slot OVER wavy/ribbed aluminum siding.
If you use double tape, expect it to ooze out in the hot weather as the pressure of the clamp squeezes the softer, hotter butyl. Every few days you run a knife around the window to remove excess, sticky butyl. Eventually, you've removed so much excess that it adds up to the entire second layer of butyl tape you put on...and paid for.
My goal...
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...is to live in a trailer.