crusty lime residue on outside

While spending the winter in Tucson this past year we evidently had a small drip at the fresh water hose and didn't realize it until we started to wash and wax our motorhome. There is a crusty line going straight down the side from the water inlet. We always take great care to wash and wax after the winter trip so this is very frustrating not being able to remove it. What will remove this stain without ruining the finish on it. Any help is greatly appreciated.