I see lots of this. The vented crawlspace is the problem. Lay down
100% plastic on the ground, overlapped 12" at the seams. Close the
vents and install a ducted dehumidifier, Run a duct from the
dehumidifier to the far reaches of the crawlspace so it dries out
You can try skipping the dehumidifier to see if the floor will go down
without it. You may also have rotted wood in the crawlspace. Make
sure the dehumidifier drains outside.