Replacing wooden core of exterior door

[habsfan]

Just bought a '96 Coleman Sante Fe in very good shape except for the exterior door.  It was sagging quite a bit so I went to tighten up the screws on the piano hinge and they all twisted freely.  I popped out the screws, then popped of the exterior aluminum trim and discovered that the wooden core was completely water logged and rotten.  My question is what should I replace the wood with?  Has anybody ever had to do this repair?  Things to be aware of?  Problems?  Suggestions?  Thanks in advance.

[habsfan]

This is what I did to replace the wood core. PT is expensive and I didn't need all the extra so I bought 2 precut pieces of regular plywood, 1/4" and 3/4" at Home Depot. Laminated them with exterior wood glue. Cut to size. Put two coats of post treatment used for direct contact in wet conditions. Done.