Before doing anything, you need to figure out what your floor is made of. I had a drain leaking under the shower. The connection just came a little loose and needed tightening up, very simple. It does that about once every couple years. I figure its from the movement of the camper on the road and use.
Anyway, I read your post earlier today and when I was under my shower jsut now to fix the drip, I took a second look at my floor and how its constructed. It's NOT what I thought at all.
Mine is a sheet of linoleum on top of a 1 1/4 inch styrofoam sandwiched between 2 - 3/8 inch plywood sheets. That is laid on the cross beams, which are laid on the main frame.
My camper is all aluminum framing, and all the walls are styrofoam sandwiched between paneling and outside luma. So, basically, my camper will never decay because of wood rot. There simply is no wood to rot!
I have a soft spot in front of the kitchen sink, that's about a foot square also. I know it's not soft because of water. I do now know, it's soft simply because of my 230 pounds standing on it! But it's far, far, far from breaking, collapsing, or failing, and definitely not in need of any replacing.
While I was there, I took some photos, namely so I can watch for any more problems with the drain pipe leaking (I cleaned up the pipe too) and can have a comparison for future reference. I like to document everything, especially with photos as the time and date stamp is on each one.
Meanwhile, here's what my floor actually looks like under the shower.
First photo is to give a bit of perspective, second photo is looking inside the hole right under the shower stall door.

