If the mark is just on the surface, you might try a Magic Eraser, assuming the side is fiberglass. If painted metal, something like McGuiar's or Mother's might take it off. Both of these are available at most automotive paint jobbers, like DuPont or PPG.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.