have a 1988 30 weekender. Should all batteries be of the same group?

I don't know about Group but I would want what ever size battery should have been installed .... reinstalled if new batteries are needed.

I don't think you should mix battery types. AGM vs Flooded.

I know my onboard charger has a selector switch on what type of batteries are connected.
 
A "bank" of batteries refers to one of more batteries being hooked up in parallel (it could be in series, but that would only be in a system larger than 12V) to effectively become one, large battery with more reserve capacity. Is that what you are asking about? Yes, those batteries should all be of the same type and group size.
 
Group typically refers to size, shape, dimensions, post configuration, etc. For a starting battery, group number can impact CCA and for a deep cycle, it can impact Ah. So no, the batteries in a 3 bank setup do not all need to be the same group size. I would let their job dictate the best group size. For example, if 2 banks were for cranking the 2 engines and bank 3 was for house loads, I would certainly not worry about them being the same group size.
 
Just to be clear - so it's not confusing, I was referring to batteries WITHIN a particular bank. Those should be the same. But from one bank to another, they may be different. And, I agree with Wylie regarding "let their job" dictate type (starting/DC) and group size.
 
My last boat had 8D starting batteries for the Cats and 6, 6 volt golf cart batteries for the house/inverter. Different "types" but separate banks. All were charged by the same charger.
 
I was thinking of using gr25 for port and two gr 27 for house and starboard. Switch is a off 1 2 also have a emergence cross over. Still have original sea ray charger in use.GR is group rating but I don't think they use that any more. Manuel calls for 105AH and GR27.
 
I use group 27 batteries on my 88 weekender.
 
I used group 27’s on my old 300 Sundancer. The last few new ones I bought I got at Walmart for about $80.00 apiece and they we’re fine. Then again, I never kept a battery for more than 2 years. I replaced one bank one year, and the other the next as routine maintenance because I like to spend time on the hook, didn’t have a generator, and didn’t want to have to worry about batteries.
Port motor had 1 with on/off switch. It only powered port starter
Stbd. motor had 2 and powered the house (all 12 volt stuff) with a 1/2/both/0ff switch.
I always ran the boat with the Stbd. switch in the both position.
My routine was to drop the hook, then select either 1 or 2 (alternated between trips) to run things. I’d put it back in the both position when it was time to start up and head back to my dock.
I spend pretty long days on the hook and never had a problem.
If for some reason I were to kill a Stbd battery (never happened) on the hook I would have the other one to rely on.
If by some crazy chance I killed both I could use the emergency start to tap off of the port battery to start the Stbd motor.
 
I have installed a 3 bank charger and the instructions indicated that all the batteries should be of the the same group. I do not have a generator on board so the charger does the work for me. I have the leads hooked to both engine batteries which also works as house batteries and I have installed a double set of deep cycle batteries as independent house batteries with a inverter to run a 120 volt fridge. All these batteries are different brands and ages but are all group 29, so far all are working fine. I also have a 50 amp solar charger that I hook up to the twin house batteries when on the water.
 
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Sounds like you have a well designed system.
 
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