Robert, the floor is 2" thick fiberglass and composite and you are correct in believing it should feel solid. It cannot rot so you can eliminate that possibility. I have a Wright-Flyer model but the kitchens are the same and have never had any soft areas. There are several cross beams under the trailer that run between the frame rails and under the floor. I guess it is possible one somehow came loose or was never installed at the factory. You can cut a small opening in the underbelly cover and take a look. I know from taking my Wright-Flyer apart the floor spans large areas between support beams without issue. Let us know what you find.
Seasonally in the Grand Marias, MN campground
On the beautiful shores of Lake Superior
2012 Ram 2500HD CTD