Kurious.G you will be happy to know that your floor *should be* 5/8" exterior grade tongue & groove plywood floor. This is what is listed as standard in the brochure for the 2005 model year. I myself purchased a 2005 31B this spring and have replace a portion of the floor exactly where you have noted as well and I can confirm that mine was plywood.
My damage was not too severe and appeared as a result of a seasonal parked camper that was not re-caulked frequently. --Definintely figure out where it was leaking as mentioned above. The bottom-horizontal outside trim piece of the slide unit tends to leak if not sealed well too.
(If you do a g00gle search for 2005 Dutchmen Brochure you will find a color .pdf from the Dutchmen website)