I see there are a few posts relating these two conditions. A few days ago, I came across this article:
It strongly suggests, to my mind, that mental stress, possibly acting on those with a genetic predisposition, causes the damage leading to RLS.
I particularly note, that besides saying three-quarters of Vietnamese veterans with severe PTSD, also have RLS, it also says: "Doctors do not understand how PTSD, RLS, and PLMD are connected".