Without seeing your floor it's kinda hard to make a diagnosis, but, I can offer what I found in my Bighorn.
When my rig was built they had to remove some of the laminated foam on the underside of the floor to run some plumbing.
It wasn't always a soft spot so I'm guessing that over time the floor was stressed and now it's soft.
I cut an access door in the coroplast, did a little measuring of the factory cutout and screwed and glued some support wood.
So now my floor is a little stronger but now it squeaks.
That's all I have to offer.
Peace
Dave