I have a '12 28ft trailer with 1 big living room slide. My kitchen table is towards the back of the slide. I have a soft spot there too.
It comes down to the the way/method GulfStream used to waterproof the bottom plate. Where they used a full wrap with the main floor decking, they only used some type of tar like water repeleant paper on the bottom. (Full wrap means they used a plastic like material completely covering the bottom, and is tacked to the top of the floor > 2 inches from any corner) . The side of the OSB used as the floor plating was not sealed or wrapped and water got into it making it to delaminate/expand causing softness. Look in the attached youtube video, they used a wrapped floor, but the one they took out wasn't. Bill Earl and Dale make a living fixing Gulfstream's poor building practices.
Why do I know so much about this? It is happening to me now.