I had taken photos of the whole process of taking my door apart, and where the screws were, but I decided it wasn't worth posting that whole part of the process and I didn't keep the photos.
But yes, the screws you see going through are what anchor the aluminum frame to the wooden door frame. Then there are two screws in each corner of the aluminum frame holding it together to itself.
So you've got a wood frame, in my case stapled together, framing some crappy styrofoam insulation, sandwiched between a
fiberglass skin on the outside and metal skin on the inside. All boxed in by the aluminum frame. At least on my 91. The wood frame is basically a 4-piece frame, top, bottom and sides, but that oversimplifies it, since there's more framing around the lock/handle. It's all pretty basic, but taking photos as you take it apart, especially at the handle, will save later frustration.
I will say though that feeling spongy from the inside doesn't necessarily mean it's rotting and needs to be rebuilt. Of course I don't know what this type door should feel like new, but mine gained only a very small amount of rigidity after the rebuild. It's just not a solid door.
Mine also had weak, poorly applied adhesive between the skin and insulation.
If you're even remotely handy, it's worth opening it up and seeing what you see. I do a lot of projects but had zero mechanical, construction or household repair experience growing up. My work is pretty rudimentary, but I'm not a complete idiot so I can usually figure things out. The door is pretty simple. I just took it off my camper and started taking out every screw I could see. How it was constructed became clear pretty quickly.