The brochure spec for that trailer is 1770 lbs. I'm not sure of the GVWR, but as Real-Lite said, it should be about 2500.
You can find the 1983 brochure here: Sunline Coach Owner's Club - Sunline Owner's Files - 1983 Sunline Travel Trailer Specifications
I would recommend using an actual RV/mobile home roof sealant for aluminum roofs instead of flex seal. I too am not sure how Flex Seal will stay bonded over time. I fear that it may lose adhesion but look fine on top, and water will seep in underneath it. With the RV sealant, it cracks when it goes bad, so you know there's a problem. Also, I know they say about using Flex Seal on gutters, so it must be somewhat UV resistant, but I'm not sure for how long. And unlike a gutter, you really don't know when it'll start to leak again. This is what I've used:
Dyco Shield and Seal 890 - Gallon - $55.20
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
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