How do people fasting for over 1-2 weeks who get very little potassium etc keep their levels normal? or will their levels not be maintained? if not maintained isn't that dangerous?

The body is a marvelous organ in preserving itself even during such conditions by holding on to what it needs and discarding others. Fasting- if total- for that long is detrimental for health

The amount of potassium (K) in your blood is just a tiny fraction of the K inside the cells which accounts for most of total body K. Besides cellular K is the reservoir, kidneys also effectively conserve potassium by minimizing the amount of potassium loss in the urine with less K intake . Low K causes serious health issues and the most dangerous ones are cardiac arrhythmia and muscle paralysis.