How to repair sub-floor in popout

As you remove material, please post some pictures. Are you sure the roof and sides are weather tight?

Very likely a water leak and it could be a bigger job than expected. There are many posts on floor repairs, search including the archives.

Underneath, you will find the waterproof mesh covering. Cut open the area where the floor is soft. Install stiffeners (aluminum 1X2). Instead of replacing the fiberglass insulation, use 1" foam board insulation, glueing it in place. This will give you 2 things...a backer for the mesh, and better R value insulaton.. Use Eternabond tape to repair the section of mesh you removed (Or replace it with FRP).

Take a look at this guy's video. He did a similar repair to his 5th wheel.

https://www.loveyourrv.com/rotted-rv-slide-out-floor-repair-fixed-manufacturer-flaw/

Here's a link to the floor replacement project I did on my slide out. Had the same symptoms, when I pulled the carpet I found that the factory used Reflectix insulation under the carpet and that was blocking the moisture from the leak from wicking up into the carpet, so the leak went undetected much longer than it should have.

I found that the Darco wrap (the black plastic tarp-like material) was letting water in but not out, so the wood framing on the slide and the edge of the main floor never dried out and just rotted away like a science project.

This may sound like a lot of work, but if the dealer opens up the project and finds what I found that $3k can go to $6k quickly....

Yea my floor has the thermal blank on top and bottom. Sounds like a very bad design to me. Thank you for the link.

Check out the link in my signature to the Lance we rebuilt, about the only thing that wasn't rotten on that was the slide...I have a fair amount of experience on these repairs, if you decide to DIY the repair I'm happy to provide some long distance assistance. I'd come over and help you if you were closer!

Thank you

Moved to Tech Issues forum from 5th Wheels.

On my class A we replaced the entire slide out floor.. NOT as hard as I thought it would be.. I started by removing everything bolted to the floor.. Then Rachael (Mobile RV service tec) came and we worked the old floor out and new one in.