I think he has batteries that are 7 years old. Doubt he has 7 batteries - more likely 1 in each bank or possibly 2 house 1 start.
What you are measuring makes sense. Both batteries(or banks) are wired to the switch in the #1 and #2 positions. Whatever you select is output to the house loads as well as the starter.
The 11.8 volts as posted sounds ready for replacement.
Your measurements were each battery individually and the 12.2 was the batteries combined, the bad bringing down the better one.
A battery that is not on charge and is resting is considered fully charged at 12.7 volts, slightly higher with some. 12.1 is about 50% charged.
If you are on a mooring and do not have solar to keep the batteries fully charged Agm batteries are not a great idea. They like to be fully charged as often as possible. An alternator, even with an external Balmar reg will not do this as the batteries internal resistance slows current to a crawl for the last 15% of so and it takes many hours. A 40 or 50 watt solar panel will top them up and keep them fully charged as you are not on shore power.
On many boats I see the switch is left in the both position. This is the fault of wiring the charging through the switch, common on many boats but not without its issues combined with operator confusion. You need to be on both to charge both. If you forget to switch to #1 or #2 after charging you will drain both batteries. And if you or someone else switches to or through off with the engine running damage to the alternator diodes will likely occur.
The solution is to route all charging to the house bank directly - it is usually the largest bank and needs the charging more than a start battery. The start battery is charged by either an ACR or Echo Charge. These are a bit different but will both automatically allow the start battery to be charged without any input from you. The 1/2/both/off switch then becomes a "use" switch only.
The best value for battery replacement would be 6 volt golf cart batteries - flooded, not Agm - in series as a house bank and a 12 volt start battery. This could be used in several ways. You could start the engine with the start battery and when anchored switch to the house bank or alternatively you could use the house bank for everything and keep the start battery as an emergency battery.
As far as the tach if you look in the Balmar regulator manual (if you don't have it you can find it online at Balmar) it will show the regulator wiring and where the tach is wired in.
Way more than you asked for but I couldn't sleep.