Hello Gazza,
Just listed a 12 volt leisure battery for caravan or campervan, and I couldn't find a precise place for it (put it under "Vehicles/Caravans & Campervans" in the end.
I suggest you need:
Cars & Vehicles / Caravans & Campervans / ACCESSORIES / LEISURE BATTERY
The items in capitals would be new ones. Other accessories for caravans include WATER CARRIERS, WASTE CARRIERS, WHEEL LOCKS and HITCH LOCKS.
Please excuse me if these exist somewhere already, but I spent some time unsuccessfully searching for them.
Cheers,
karaval.