I agree with BCK that this needs careful investigation!
However - after you determine that it is indeed dry now and /or fix the source of the water.
And after you check to make sure you're only dealing with a small spot, you might check out some of the epoxy products developed for handling dry-rot in the wood boating industry. They make epoxy compounds that soak into wood and replace the rotten part, bond to the good wood and make a really solid fix. But it only works IF and only IF the wood around the fix is solid and doesn't continue to rot. It takes a bit of care and then some sanding to get it right but it can be much cheaper than pulling out enough to replace a section of floor.
Buddy Ray - Atlanta
Jayco 2016 Eagle HT 26.5RLS
Ford 2016 F150 Lariat, 3.5L V6 Ecoboost
Max Tow Pkg, 36gal tank
Reese Sidewinder and Reese Titan 16k hitch