Wow, I never knew that they built lightweight frames like that! Looking back at the first pic, I'll bet what I'm seeing under that splice plate isn't a tear, but the lip where the edge is rolled to stiffen it.
Here's a pic of what I was trying to describe earlier:
What you're looking at is the front edge of the slide in my 2008 Terry. The project at hand was to replace the floor in the slide which had begun to rot out due to a leak in the exterior trim. The carpet in the slide has been removed, and you can see where the beveled edge of the slide floor now sites on the remaining carpet that needs to be removed.
With the interior fascia trim removed, I screwed a 4x4 block to the front framing on each side of the slide and used a bottle jack to raise the slide in the opening so I could get under there and get that carpet out.
This is what I was thinking for you as a way to support that wall when you are rebuilding the floor under it. The easy way would be to screw a 4x4 to the exterior but then you have to deal with holes in the fiberglass, that's what made me think of removing a window and putting a support there that you could jack against. What ever you do will have to be on the exterior, there's nothing left to jack against inside.. :)
Now that you know the frame is good, nothing left to do but to do it! Post back pics once you get into it so we can all share in the fun!
