If that doesn't work, try an old standby - Wesley's Bleche White (honest, that's how it's spelled). It's a whitewall tire cleaner that you can find at an auto parts store or in the automotive cleaner/wax section of one of the big box stores.
Be careful with it, though - it can remove wax, and I don't know what it would do to paint or decals.
2016 Ram Longhorn 3500 Dually 4x4 CCLB, 385/900 Cummins, Aisin AS69RC, 4.10
2014.5 DRV Mobile Suites 38RSSA #6972
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