The states which use Reserve America, and federal campgrounds have an availability view which you can use to see which sites are booked. While hosting at a federal CG I've been able to use that view to get a good idea of how busy we would each weekend a couple days before things would show up on the DAR.
If you are wanting to get into a popular campground, start checking that view a couple weeks before your desired date to make sure you are familiar with how the bookings go.
Waiting until the day you need to make the reservation only results in frustration.