Are you referring to the wood in the attic what supports the roof AC unit? If so, you can use the normal wood studding at the lumber yard and cut it to fit. It is soft wood, will have a stamp on the side called, SPF (Spruce, Pine, Fir). If you are getting any plywood, again it can be normal plywood, just look for exterior glue ratings. While this is not directly outside, the exterior glue will hold up longer in the higher moisture environment of living outside in the winter with no heat in the camper.
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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