Theres a showground at Princetown... not sure if its free but it looked pretty basic. When the Gellibrand river mouth becomes blocked by sandbanks that area floods. It becomes a beautiful wetland.
There are very few camps with no charge in along the Great Ocean Road - especially for a rig your size. There is Joanna Beach which may be a possibility but doubt there would be sufficient free space for your rig. There are also Aire River East and West but they are smaller than Johanna Beach.
There are few camps in the Otways, but these are even smaller and most have some fee attached anyway.
I would suggest you ring ahead even if you are going to a van park - they have been there for years and not necessarily with large sites for big buses.
Princetown, as suggested may be OK, but the bridge in has a 5tonne limit ... ring ahead as this is peak period and it is likely (as are all the sites) well partronised.
Cheers - John
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2006 Discovery 3 TDV6 SE Auto - 2008 23ft Golden Eagle Hunter Some people feel the rain - the others just get wet - Bob Dylan