Our 2011 Fuzion has this same problem.
We just found a weak spot and read your post and pulled up our flooring today to reveal the floor rot... Not sure where the water is its coming from yet .. thinking its a window since ours is on the right side only.
update: So it looks like silicone/sealer was over looked when the rear door awning was installed about a 6" section was open without sealer of any kind... that was not the only source, another section at the back didn't have any sealer - will post pictures of that.
Anyone with a 2011 toyhauler from Keystone should check these seals at your ramp door...
Or camper is stored in a shed, we use it one week a year! This is so shocking that this was overlooked...