Correct Ben, most are Weyerhaeuser Structurwood OSB. That was started sometime in the early '90s, and the benefit is that Sunline could buy it in large pieces. Something like 25' or 30'. So longer trailers were two piece, but the majority of Sunlines built have one piece floors.
Provided it doesn't get wet, they seem to hold up pretty well.
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