My father-in-law is on dialysis and also has to watch his sodium, potassium & phosphorus intake. The dietician at the dialysis center gave me a hint about potatoes - both white & sweet potatoes. You can reduce the amount of potassium in them by "dialyzing" them. Peel & dice the amount of potatoes for the person(s) needing to watch their potassium intake. Place the potatoes in a large container and cover with water to where the ratio is one part potatoes to ten parts water. Soak for 2 hours. Drain & rinse potatoes. Place back in container and cover again with 10 parts water. Soak again for at least 2 hours. If using white potatoes, you can drain, rinse and cook as you wish at this point. If using sweet potatoes, you need to drain, rinse and soak for at least another 2 hours before cooking as they contain more potassium than white potatoes.
Also, for those needing to watch phosphorus intake, root beer is allowed!! The dialysis center has said that is a "free food".