Our motorhome had the fill on the left and the gas refrigerator with its flame on the right. In the case of such a vehicle, fill on the left is vital as there is no way you are going to reach across with the fill hose. That tells me that gas station traffic should really keep right as on the road with filler caps on the left of the vehicle.
It would be good if fillers on the left were standardized but it is not vital for a car.