You are not alone! Almost all Sunline's I see with square windows have this problem. My 2251 had lost almost all of the trim and had to be replaced. I turned to the guys at All-Rite. They have your trim. If you can salvage a piece off, a cross section should look like this:
It's called the Rigid Screw Cover, available in 8' Sections of Black, Brown, Polar White, or Colonial. Since yours is a '82, I'd personally recommend the Polar White, since your exterior is more white than the '83's dark tan. I also think the Colonial would look good, depending on how dark it is compared to your coach.
Here's the link: http://www.all-rite.com/
Last time I checked, All-Rite couldn't help me with the trim for around the entry door and compartment doors. On their website, it looks like they now carry the right piece, but I'd check with them first if you need some. It looks like it could be the Phillips Screw Cover.
Remember, these parts are based on what I found for my '94, so the parts may differ for a unit 12 years older.
P.S. The correct model number for your Sunline is not T-1550. The model number 1550 didn't come into effect until 1983, when Sunline went to all numeric model numbers. Prior to that (including 1982), the model number was 15 1/2 SB (15.5 SB as I call it), with SB meaning either Super Bath or Super Bed. The meaning changed between years and models, so I don't know which one it was.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9473.8 (as of 6/18/21)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR