Sorry but this is perhaps the main drawback of buying privately especially as it was a transaction based on "sold as seen"
Buying through a dealer you would probably have paid more but would have had some form of warranty, even a 3 month warranty would have helped.
Your only hope (very slim) would be to appeal to the sellers good nature and ask for a contribution towards the cost of repairs. More than likely they will not oblige and may say the cost of repairs was already factored into their selling price -------Buyer Beware applies I'm afraid.
your lucky we have a 2004 470/4 and it had cracks on the back pannel i have filled them and thought ah well put it down to experiance ( first ever caravan) warrenty was only for 3 months,
after a winter in storage we noticed the shower pannel was loose at the bottom sw me and father in law said no problem we will take it off and glue it and make a good job
then shock horror lots of tears and yes it was black and soaked, first thorghts was well 6000 pounds is now worth nothing but father in law is a joiner so knows wood just said not a problem so its now all drying out and its not as bad as we first thought it just needs cealing when completly dry as its only the 3mm ply that is effected.
so get them cracks sorted and hopefully you wont get what we went through