I had the same problem. The soft spot was under the back corner of the slide. Right where the slide met the wall. I pulled the slide about 3/4 of the way out. I got under the slide and slid a 1'X 2' 14ga steel plate between the carpet and decking. I bought the pre-cut plate at HD. It wasn't easy but I got it in. There was a black fabric material between the decking and carpet that gave me problems. I filed the corners and edges of the plate smooth so it would slide in easier without catching on the carpet or fabric.
About a year later the floor deteriorated enough that it became a large soft spot. Even after the leak was fixed. It cost me $2800 to have it done right 12 years ago. I have no idea how much that would cost today. I do know they had to pull out the slide to fix it.
2011 Ford F-150 EcoBoost SuperCab Max Tow, 2084# Payload, 11,300# Tow,
Timbrens
2013 KZ Durango 2857