Well there's Walmart; they'll keep the lights
on for ya. As Bob reminds, planning one week out is not optimum. We reserved for our "week beginning" visit about three weeks out. Had a choice of five sites with electric and two loops of dry trailer/tent sites. At closer range time wise, sites with juice were unavailable Sun/Thurs. as well.
That's a time share at the old watering hole: the herds and hordes tied to school and work are going to go for the magic three day holidays; folks with a bit more lenient schedule can consider themselves kings of the campgrounds if they choose to wait until the wildebeasts have come and gone.
Impromptu x-country trips composed of one-niters probably isn't anyone's ideal camping trip. The model for that is long haul trucking and your beautifully-appointed parking space is most likely at Flying J.