In defence of the Seisia Caravan Park, we arrived there just after all the Queensland School holiday rush had finished, and found the park to be a great spot to make camp and explore the surrounding area. We booked into one of the hut areas, which are located right on the beach. Maybe assess whether to stay there considering what stage of the school holidays you are at.
The Falls camping area is definately first in first served. Find a spot, set up camp, and then think about payment.
There are plenty of bush camping spots near the many river/creek crossings, so assess these as you go along, and plan to pull up early in the afternoon if in peak period, to ensure a good camp site.
As you would know from your previous trip, travelling along the OTL is not as hard as everyone goes on about, it's only a couple of the creek crossings which make it a little more interesting, especially so with a camper on behind.
Our trip up there in '10 was limited to a month from home to home, and so we lost a lot of time on the bitumen just getting up there. Make sure you allow plenty of time to poke around some of the out of the way places, whilst we had a good trip, there was plenty more we could have seen. Our trip was more about the journey, then the destinations, and just making our kids familliar with all the places I had spoken about from previous trips.