I've not sold a caravan whilst on finance, but I've sold a couple cars whilst on finance (due to low part ex values) so maybe the method I have used that may be helpful:
1) Get an settlement figure and the finance companies approval about the private sale
2) Advertise your Caravan for sale
3) Declare that there is finance outstanding to any potential buyer
4) If sold get the buyer to make a bankers draft payable to your finance company and the difference payable to you
5) Once the cheque has cleared, Get a settlement letter from the finance company and send a copy onto the person who purchased the caravan for their piece of mind.
I've always found that by being honest with all involved, I never had a problem.