I was able to take a look inside the rocker panels by removing one of the rubber plugs on the back side, shining a flashlight into the panel and looking in from the square holes on top where the scuff panels screws in. Thanks to starquestman and tortron for pointing out those openings.

The good news is that there is no rust in there, so I don't need to worry about threatening that area. Just need to figure out a lasting solution for the surface rust issue on the outside that keeps coming back. I think I'll treat it with the rust neutralizer again but hold off on painting for a while, until I know for sure if it worked.
I did find a lot of pine needles inside the rocker though, so I have to figure out how those are getting in (my guess is through the front, from the runoff drains of the air vent cowls next to the windshield wipers), and how to vacuum them out of there so they don't collect moisture.

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