last year when i stopped in Santee overnight i left myself plugged in all night long. but back then, there were no idle fees.
I was the only one there when I plugged in, but there were 2 other Model Ss there in the morning when I unplugged.
The in-dash Nav will tell u if there are idle fees to contend with. if there are, I would not leave myself plugged in overnight. even if in the most likely scenario the station will not reach 50% or more capacity. (which is when Tesla will enforce the idle fees rule)
if there not idle fees to contend with, i would say you are safe to stay plugged in overnight in any case.
are u hoping to charge to more than 90%? if yes, and you do have idle fees to contend with, things could get tedious.
That said, you can make it from Santee SuperCharger to Savanna SC easily without charging all the way to 100%. In fact, the distance is only 110 miles or so -- you could probably be ok leaving santee with a 50% battery and have plenty of electrons to spare to make it to Savanna SC.