Just sold a swan and went through this. If you find and download the tranfer ownership form from the vicroads website, the 1st 2 pages will tell you what you need.
From memory there is a rwc exemption for caravans and trailers under 4.5 tonnes.
The same exemption applies to stamp duty.
The buyer sends the form in and the tranfer fee in $5.90 from memory.
Its all on the transfer ownership form which you need to fill in and sign.
Hope this helps.
Ras
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