Hi Sarah.
The underside of the floor in the corners get a bunch of junk from the road on it from the wheels. Mine had the sAme mushy feel when I stripped out the interior.
I had a support bolted on the frame to extend to that area on both sides. Also reinforced with
fiberglass inside. Then I sprayed on that stuff like truck bed liner on the underside. Some people also put mud flaps on the frame behind the wheels.