Looking at the diagram, it appears that there is a permanent 12 volt supply to pins 2 and 3 of the 12S plug. I would have expected a relay across these two pins, with the contact common going to the terminal block, fuse no. 1 to the N/O contact and the 12 positive output from the charger to the N/C contact. This would protect the leisure battery from slow discharge if the 12S plug was exposed to wet conditions.