If you're travelling outside school holidays (before mid-December or after the end of January), I would keep things open and not book anything. Just put together a list of places you want to see and go through them one by one, then when it gets dark, stay the night. Some places worth visiting are below and I've included how far they are from Sydney:
Nelson Bay (2.5 hours)
South West Rocks (5.5 hours)
Byron Bay (9 hours)
Surfers Paradise / Gold Coast (10 hours)
Noosa (12 hours+)
While the Whitsundays is really beautiful (Whitehaven Beach especially!), I think it's a bit overly ambitious for a 10 day trip. You could go to Hyams Beach in the Jervis Bay area (2.5 hours south of Sydney). This beach is regularly rated second only to Whitehaven Beach as the nicest beach in Australia.
If you're travelling DURING school holidays, you'll probably find most of the coastal caravan parks will be fully booked, even for basic unpowered sites. There may still be availability in the ones away from the coast or along highways.
Anyway, why go north? The south coast is nicer in my opinion - Stanwell Tops, Austinmer, Kiama, Jervis Bay, Batemans Bay, Narooma, Merimbula, Eden.