I have used Alaska's Campgrounds for decades, best in the nation. The best, do  have some sites for "reservations" and some for the "drop in". which are generally more spaces. Now the campgrounds near the popular spots are a different deal of course My advice is simple, many campers leave the campground not as early as one may think.In fact many sleep in late. Then eat breakfast that is closer to lunch. I have found myself having the campground to myself in the early am and often on a lake or river fishing by myself. The best time to check for a camp spot is not after dinner or bedtime, although that is a popular time with travelers. When camping at a popular campground campers leave around noon so if you think that would suit your plans to me that is the best time to get a camp spot.