I could nearly bet that if the Elliot Falls sites are all booked out, that all of the popular camping spots are going to be crowded.
It will be a suck it and see type of trip,
Dulhunty Creek , a fairly easy run from the Archer River Road house in one day. Quite a few camping spots but will be busy in school holidays. Lovely creek to swim in.
Canal Creek. Is close to the Twin Falls, Elliot Falls and Indian Head Falls. And again a easy run in one day to get there from Dalhunty.
Camping spots on both sides of the creek. Great spot for a two night stop over , as you need a full day to enjoy the falls .
Nolan’s Creek . Another great spot for camping, plenty of room but again all these sites are very crowded during the school holidays. from Nolan's it is an easy run to the tip in one day.
I have listed a few great spots, and there are many more good spots along the way. Basically if you can find spot to camp grab it as the next site may be full.