At Big Mac, there is no need for a campsite, just find a spot on the beach and it's yours. Wide open BLM type camping. The nicest sand beaches you can find without going to the ocean.You have to pay a camping fee ($7/day), but you can camp where you want. There are organized campgrounds, but all the years I've been going there(about 40), I've never stayed in one. There is also a daily park fee of $4 or you can buy a yearly pass for something like $20. Cross the dam and camp on the north side of the lake. The farther west you go, the less crowded it gets. I would recommend Arthur Bay and then go west.
I was there last weekend and the lake is as full as it's been in quite a few years.
Forgot to add, NO alcohol. Just keep it under wraps, drink out of a glass and you won't have a problem unless you get real rowdy. You will pass through a pay booth, so make sure it's not visible in the vehicle.