Here is what I found. This is the floor in a 2004 T-1950
The actual dimension what I found is 0.550" OSB. That could convert to 14mm thickness. It's more than 1/2". And I found you can get 14mm OSB, just maybe not at the local Home Depot etc.
It seems to thin to be the declared 19/32" OSB which is actual 0.570 and it shrunk some over the years.
At that time in 2004, Weyerhaeuser made what was called Structurwood and I seem to recall that was what Sunline used in that time period. They could buy it the full length of the camper, long too, which means it had no seams. That brand Structurwood I believe has now been replaced with the next generation OSB something.
Not really sure what your 89 camper has. But even if you buy 5/8" nowadays, it is not the old 5/8" it's something thinner.
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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