I was there last week. It wasn't busy. Dont bother calling the number listed is to a regional ranger park office and they have no information on the campground.
Sites are FCFS (First Come First Serve) and cell reception at the campground is impossible, but since the recent minor expansion (primitive sites on side B), they very well may have a site, but if you're wanting electric, i hope you get there in time since your asking about a holiday weekend.
I would call definitely to check if they have any openings.
Thanks! Your answer is awaiting moderation.