Its not really possible to accurately price a caravan of that age. In reasonable usable condition with a dry interior, ie no body leaks, possibly £300-500
Your best bet would be ebay, reaches the widest audience, set the reserve for minimum you will take for it(but be realistic)& hope 2 bidders both want it as much as the other. Or you can put it on ebay as a classified ad. Not a good time to sell a caravan now though as its the end of the season. If you can keep it until about April/May & sell it then you should get more for it.