Well I just uncovered some spongy floorboard on the corner of my side out. This was in an area I have repaired a leak on some flashing a few years ago https://www.loveyourrv.com/leaky-rv-slide-room-repair-reseal/
I've been watching that area and lately more has been going soft.
Doesn't look like it's coming down the walls. I think what is happening is during heavy rain the water runs off the slide-out roof, down the wall and then curls under the bottom lip and has slowly over the years wicked through the plastic covering and settled into the plywood, thank god it is plywood!
We tend to park a slightly nose high so makes sense the back is getting it and not the front.

I luckily have lots of good floor board left, So, I think my easiest fix is to remove the damaged wood area. Then add a piece of thick plywood to the top floor under my daybed. This will give me something to screw into from below and add a new piece cut to shape. Maybe screw and glue it on.







I think I had the same thing going on that this guy explains