Check and see that none of the electrical components are draining the batteries through faults.
I agree and best way is to install a victron bat monitor. Does wonders. Basically on shore power and bat bank us fully charged and still notice the there is more then few Amps current fed into the service bank while all equip is off, then you got a leak in the system or worse a bad battery internally shorting and sucking current constantly. As soon as you unplug from SP, your good battery will also drain. Check also which battery us getting warm.
Victron also shows you how much u consumed while sailing or at anchor and set an alarm.discharge your battery 12.5. The secret for long lasting lasting batteries is keeping the above 70% level and charging back to 100 as soon as possible.
Totally agree and do install A2B sterling which will also save your batteries on long trips especially ur starter battery. Well worth the money as stated above.
Size up your bank according to ur demands. We had someone here with 2 x 140Ah bats and shot them in less than 3 yrs. He thought seeing 12v was still ok ank kept on doing this for many weekend trips powering fridge freezer etc...
Less you discharge more cycles you'll have. Average batteries (non deep cycle) will be able to cycle 400 to 500times before it bocomes dead weight. Why not extend its life by 2 or 3 by NOT letting discharge below 12.5v