A Sunline has Aluminum sides if the siding is rippled and has a rough texture to it.
If it has fiberglass, the sides will be smooth and shiny.
IMHO, fiberglass is better than aluminum because it is easier to clean and will last a lot longer. The one downside to fiberglass is the weight. I don't know specifics as to how much more a fiberglass unit would weigh over an aluminum sided unit, but it is enough to worry people with small TV's.
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