Welcome to the forum. The vinyl trim you're asking about is called "Screw Cover Trim". It's available at any RV parts store, on Amazon, EBay and many mobile home/travel trailer parts houses.
Here's an Amazon link to a number of different products that all do the same thing. Your aluminum trim is either 3/4 or 7/8 inch, so you'll need that size vinyl trim screw cover. The two sizes are interchangeable, so the 7/8 inch will fit both.
You can buy a 25' section for around $7 or a 100' roll for about $20. I'd recommend you opt for the larger size roll. If that section of screw cover is already showing signs of UV damage, the rest is probably not far behind...
https://www.amazon.com/s?k=rv+screw+...refreshed_4_19