For anyone brewing with RO or distilled water, the number one deficiency is likely to be zinc. Zinc is present in most water supplies at low levels and is typically sufficient for yeast nutrition. However, the RO process reduces that by at least 90% and that leaves the water too low in zinc. The other trace ions seem to be less important for the brewing water or they are supplied sufficiently by the malt.
One option for supplying zinc, is to use zinc sulfate heptahydrate. That mineral is readily available as a nutritional supplement from healthfood and vitamin vendors.
It turns out that ale yeast prefer more zinc than lager yeast. The dosing rate for ales is 3.2 milligrams of the zinc sulfate heptahydrate per gallon of wort. The dosing rate for lagers is half that (1.6 mg/gal). As you can see, these are exceeding small doses and they require exacting measurement since you don't want to overdose the zinc. It creates a metallic flavor at pretty low concentrations. So, DON'T overdo it!!
The other ion that you should include in your brewing practice is copper. It is needed by yeast to scavenge sulfurous compounds from the wort and beer. All you need to do to supply this ion is to have some copper metal somewhere in your brewing system so that wort comes in contact with it. If you have all stainless equipment, I recommend placing a short length of copper tubing in your boil kettle and that should supply all the copper that is needed. Don't worry too much about overdosing with copper since the yeast remove virtually all free copper from the wort and the beer will be almost copper free.