The floor would be 1/2" thick OSB sheet ( oriented strand board). They did not use plywood on the travel trailers on your vintage camper. In most all campers, the floor is all one piece, back of camper to front. The OSB sheets where made that long to accommodate.
OSB does swell when it get exposed to moisture. And once swelled, even if it is dried out, it does not seem to return to it's original thickness.
If you are doing a repair to part of the floor, you can use plywood or OSB. The thickness may have to be shimmed to to align to the tie in of the original floor due to the swelling. That and the fact is is hard top find true 1/2" thick plywood or OSB. Many times the lumber yards have metric thickness which is close, but not an exact match up.
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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