Hi Yumasgirl. Welcome to the Forums!
The plastic covering the edge of the metal sheets gets brittle and breaks. I found that Oasis RV
in Tucson had some and they have very reasonable shipping rates. They called it J-trim.
You also could try Inland RV
(California) or Out of Doors Mart
You do have to remove rivets at the edge of sheet metal, re-drill through the new J-trim and put in new aluminum pop rivets to replace these edges. You have to cut the hidden part of the J-trim about every inch or so to make it around the curves by the window (leaving the visible part intact).