All new and many older caravans have outside lockers for the battery, it's got to be very old to be stored under the seat. Check the outside locker near, or backing on to, where the charger is and you may have missed seeing the two leads. Try the Windrush caravan club (Obviously for Windrush owners) telephone 01235 831879 you may have to join though £15 per year according to the website.
www.windrush-caravan-club.co.uk.
bob
May 2008