Odds are good the original wood was just plain pine or maybe fur. Depending on how many years your looking for the door to last, you can use the pine, popular or most any other wood. Key is to seal out any water the best you can from getting to it. If you want the real added protection, pressure treated can work too. The difference is the cost.
See here, Frank_a rebuilt his Sun spot door. It's about 1/2 the way down the page
Good luck and if you can, post pics of the before, during and after. These pic posts really help the next guy coming behind you.
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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