I had something like this on our coach forward and read of the passenger side rear tires. It washed right off and I attributed it to road oils like TJ suggested. We had been at the factory so traveled through Indiana and Ohio road construction sites. It has not returned since, so I don't think it was anything other than road grime.
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Bill and Dustee and the Kids
2018 Newmar Bay Star 3406
2015 Jeep Grand Cherokee Overland
FMCA: F482061 | NKK: 22422
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