No need to be sorry. I feel your pain. Sounds like the previous owner delivered a real doozie to you :-)
OK, what to do?
1. Just replace the battery monitor and forget everything else for awhile; or
2. Think about a more rational arrangement for your batteries.
The first is relatively easy. The second isn't. It requires thoughful consideration of the existing system, and of how you intend to use the boat.
Are you going to cruise
the boat? Stay mostly at dockside? Stay mostly on anchor
What kind of charging
devices do you have aboard? Alternator
internal or external)? Solar panels
? Wind generator
or gas generator
? Sounds like you do have a generator. Size? AC or DC?
IMHO, it doesn't make sense to have a separate starting battery for the engine
and the generator. One will more than suffice, particularly if it can be supplemented...as needed...by the house batteries.
Nor does it make sense to have separate battery banks for the house (two banks of 200AH each) and another bank for the freezer
(size in AH?). These could all be run from a single
house bank. In theory.
In practice, you may have some special needs and problems. Only a close examination of what you now have, how you plan to use the boat, and what you'd like to have in terms of a more efficient electrical system
will yield the answer.
Sorry to complicate what was a seemingly straightforward question about replacing the Link 20.