My limited experience with trying to secure a camp site at the Portal is if you can get there as early as possible, like before 4pm, your chance of getting a walk up is good. If not check with the camp host when you get there for any cancellations.
There are other CGs lower down the hill. I don't know how set you are on sleeping at the Portal. Another option is finding a parking spot in the trailhead/hiker lots. But if you value getting any sleep I wouldn't recommend doing that.
Good Luck
