Batteries - IP 485

First, I'm surprised to not see a Yacht System category of Electrical/Batteries or something along that line.

I recently purchased a 2007 IP 485 and it's about 700 miles away currently in LA.  I knew when I bought the boat that the batteries (Lifeline GPL-27 T) were minimal, but the survey came back and reported 12.6V for the bank, but the surveyor admitted he wasn't able to check all the batteries individually. 
He recommended having each battery "load tested".

Yesterday, my boat caretaker (who knows nothing about boats) reported when asked that the 12V voltage was reporting 12.2V.  Which is too low. 
The boat has been in a marina for the last three weeks and tied to 50A power, with just the AC/Dehumidifiers and Waterpump on (on the recommendation of the previous owners). 
So there shouldn't be anything really drawing down the batteries which makes me suspect that 1 or more of the batteries has failed or they may have all reached the end of life.

I was hoping to replace the batteries this summer after we brought the boat to Puget Sound.  But with an 1100 mile trip ahead of us, I would like to have a more reliable battery bank.

So my thoughts would be to a) test all the batteries, by pulling them out, disconnecting them, and testing, 2) identify any that are really bad, 3) replace with like batteries. 
If this is a good course of action, what should I use to test the batteries?  I've always used a multimeter but the surveyor had some specialized battery tester that I didn't recognize.

How does one "load test" a battery?

Is there another course of action I've not considered?  Replacing all the batteries is ~$2500 (my guess for cost, not labor), so finding someone to replace them would also be an option. But not my first choice

Is there something else I've not considered?  If you ask me about the battery charger, at this point I will plead ignorance as I'm still learning all the boat systems.

Thanks,

Sean

Hi Sean,

You may have solved this by now, but chiming in anyway. Certainly worth looking in to your charging system. Maybe it was off? The charger is likely a smart charger, and would be set to AGM, for your Lifeline batteries. For load tester, you can buy those at an auto parts store. They put a fixed load on the battery, not sure how much, but probably 100A or so, for a short time. And they should measure the battery voltage under that load. If the batteries are in good shape, I expect the battery voltage would only drop by a volt or so. Not sure of the Lifeline spec. Generally it is good to replace the entire set of batteries, so they share the load properly. An exception would be if all batteries are almost new, and only one failed. (this would be rare).
As fyi - I'm replacing our bank of 6 Lifeline batteries, after 15 years. Probably should have replaced at 7 or 10 years. We have solar panels that keep batteries topped up (at float voltage) even when on mooring or anchor.
Good luck, Drew