Its probably butyl tape which is a sealant applied to the back side of the window frame when it is installed on the body of the trailer. It keeps water out. The ragged edge of the sealant is the excess portion that has oozed out. You can scrape/cut it off with a plastic blade.
Eventually the butyl tape after 20 years or so begins to fail. Then the windows
begin to leak around the outer edge. At that point, the windows
should be removed, the old sealant scraped and cleaned off, and new butyl tape applied before re-installing the windows.