Fred, your English is great -- what is your first language?
If this is just a thin line of mold, then yes, this is probably from condensation. Has the trailer been stored in a warm wet area?
If it is a lot of mold, it could indicate a leak in the window frame. It is a big job to remove and re-seal the window, but it is not technically difficult. You will need two people. You unscrew the window, remove the frame, remove the window, and use butyl tape to re-seal the window.
I have done this job twice. My old T-139 had a leak, and we fixed it. Our new X-139 had a crack in the front window, and we replaced the glass.
You may want to spray a hose on the window, all around the frame, to see if water is leaking into the interior.
Good luck, and be sure to ask questions! If you do not understand what I have written, please feel free to say so, and I will be happy to explain it more clearly.
__________________
|