Well, you are lucky the access is in the pantry. I would pull out the shelves and make the entire back wall my access panel. If the back wall matches the interior, you can buy some 24" or 12" wide seam tape and make your own panel that matches your interior.
I am a bit of an animal, so this is only the opinion of a "tear it out and fix it all" kind of guy. Just don't let the water issue go unsolved. I encountered a lot of mold because of those lousy RV shower fixtures and the old style shower builds (2023). I put in an all brass residential fixture in mine. No more silicone and cheap twist on water line fittings. The pictures show, what I encountered. Because the water goes to the bottom of the wall, that is where the mold will grow first. This is why I recommend making the entire wall an access panel. I also posted the most amazing mold remover EVER!! No scrubbing and clears the mold in porous materials in 15 min.
