The original from Sunline would be corrugated aluminum siding I do believe back in 1988. The siding would have paint on the outside, tan in color for 1988, and it was bare silver aluminum on the inside of the walls. While they did make smooth fiberglass sided campers, they were later model years and there would be no shiny aluminum looking metal inside the walls if they did. They bonded the fiber glass to a luan wall panel so you would see a wood on the inside.
A few pics of the outside of your camper may shed some light on this. Since your asking, if it was fiberglass or vinyl, a prior owner may have done something over the original siding. Again, a few pics of the outside can really help show us what your looking at to know better.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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