Keith, I wrote about pH probe storage in a post on the Bru'n Water facebook page. It presents information direct from a major industrial pH equipment manufacturer and the recommendation is to use a potassium chloride storage solution.
There are manufacturers that recommend using distilled water to keep the bulb on their probe's moist, but that distilled water shouldn't be in contact with the bulb or it will osmotically draw the ions out of the probe's electrolyte. For that type of storage, you need to use just a couple of drops of distilled water in the probe cover and that creates the moist environment without the possibility of destroying the probe.