Also, there were some areas I sanded a little on my trailer.
If memory serves I started with 220 grit in some of the bad areas, and worked in steps all the way to 5,000 grit before polishing.
Don't try that with anything but a HIGH QUALITY professional pneumatic DA sander like a Hutchins, a hobby sander wont cut it.
If That stain is pitted oxidation like it looks like, You might have to start with 80 grit in order to sand off enough material to get rid of the bigger pits. That's where a white roof can greatly reduce the area you might need to sand extensively in order to make look good polished.
It wasn't important enough to me to change panels for a consistent "mirror finish" so I used the "good enough" principle and some camouflage on my project.
You can sand SOME with 220 without sanding through the Al-clad, you can sand quite a lot wit 600 grit without burning through, but do a lot of testing up high in an inconspicuous area so you can figure out how far you can go.
I would say that if you need to burn through what's left of the alclad, you will need to do so to everything above a certain height to make the trailer look good.
The trick is going to be picking the right line of demarkation.
Try to avoid sanding the rivets, they sand off quick.
The smaller the area you work, the less work you will have to do. Don't commit yourself to a giant project early on, start up high and work down until you're satisfied, if you start low, you will have made a giant commitment early on.