Storage of Gel Deep Cycle Batteries

This summer I have installed 12 new 12V 110Ah gel deep cycle batteries on my boat. It was an expensive exercise, I want to maximise the life of these batteries!
When I lay up my boat for 7 months over the winter. What is the best way to store the batteries over the winter please?
(a) Fully charge the batteries then completely isolate them?
(b) I have a Victron Multiplus 24V 3000W 70A, do I leave this connected to power and let it do its job - Storage mode?
(c) I have solar panels with a MPPT 100-50 - leave this connected? I am aware this does not have a storage mode like the Multiplus

Regards
David