The first time I got rls I saw on the net that it could be caused by low iron. Then I realized that I had run out of iron supplements (I rarely eat red meat). Once I began the supplements again the rls went away.
Then quite some months later it returned. Well, I upped my iron and it went away again.
You can't go by blood tests alone. They may tell you that your iron is fine while it is not. Go by symptoms also. I used to have a doctor, now retired, who said that what they call "normal" in blood tests sn really average. But he said a few decades ago, before soil depletion and a toxin laden world, the scores for average/normal were different.
You could be low not only in iron, but in potassium or magnesium or calcium. These are correlated to rls. Low salt diets can also be a factor. Low salt can also cause leg cramps. I was not having much salt, for ex., and got cramps in my foot. I took a bit of salt and in an hour the problem was gone.
Btw some say to sleep with a bar of soap rls and it does wonders. I tried that and it was totally worthless as one would expect.