In general it is not usually possible to make reservations at campsites, first come first served. It is not usually a problem, even at the locations you mention. Though there are some traditional party weekends, mainly the weekend containing the first Monday in August when certain camping locations, (but I am not aware those you mention are included in them, its mainly Husafell, Thorsmork and Asbyrgi), are descended upon by large numbers of Icelandic youth. But in late August, you are already out of high season and are unlikely to encounter any such thing.