Hello and thank you for your question.
Your situation is a little uncertain as the law on cooling off periods provides that a consumer transaction such as the one you describe will only be subject to a cooling off period of it is an unsolicited sale, which typically means you were approached at home or in a shopping centre and then pressed to sign a contract, but won't apply at the vendor's usual place of business. If you were at a caravan show, then a lot will turn on the exact way in which you came to make the sale. The following legal aid link explains:
Noting that your situation is debatable, if they will let you out for just the loss of your deposit that may not be a bad outcome as the alternative might be that they don't let you out at all and then sue you for completion (i.e. to force you to complete the purchase).
If you decide you do want to pursue your full deposit you can initiate proceedings in the Queensland Civil and Administrative Tribunal, and can read about the process here:
I trust the above assists.
Good luck and please rate my answer.