Okay, I assume that you want to keep the caravan?
If so, your best option is to go to your local CAB and speak to someone there and get advice on writing a letter to the finance company.
Essentially, the finance company should help you (and since it is an HP agreement and you would have paid a 1/3 of the total outstanding they will need a court order to repossess the caravan). If you can show that you are being reasonable, then the court will find in your favour.
You need to write them a letter, detailing that you are now unemployed (and give dates since unemployed) and give them a copy of an income / expenditure form (available from CAB (or even a bank)).
Tell them that you can only afford topay them �x per month and that is it.
Send the letter recorded delivery and of course keep a copy.