Battery storage trickle charger

I have a small Minn Kota charger, but it's similar to many others.
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A wet cell doesn't discharge much when it's idle,, but any periods of under-charge contribute to decreased battery life. Just to keep it up, I put mine on the charger above all winter, but I put TWO vacation-type timers in series with the charger. Both are set to pass current about an hour a day, so first sends current to the second an hour a day, and the second sends current to the charger an hour every 24 days. With two batteries you'd want to connect them in PARALLEL (plus-to-plus) with jumper wires so the charger is seeing 12 volts, not 24. Hard to describe, but that keeps the charging to a minimum, which is all that's required.

I had 4 batteries and used 2 Minn Kota 210D's one will charge 2 batteries and will trickle/storage charge all winter BPS also has a like charger which is a little cheaper.

I used this for 3 years when not using my boat stored inside. It was setup to charge my two 12V batteries hooked together. It did very well.
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You could also use something like this:
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if you don't have 110v where you store the boat. I use one when I store my boat in Fairplay out in a fenced lot with no power. It will re-charge and maintain a 12 v DC battery at about 3-4 days charging for 1 day's use (depending on sunlight amount). Get a decent one, though, not a thin-film Chinese Cheapie. HAS TO HAVE a voltage controller/regulator so it doesn't over-charge the battery. (That's true of all chargers.)

Look at the NOCO GENIUS 2X2. 2 banks, 2 amps each, smart charging. Hook it up and walk away.
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Any kind of two bank battery charger will do the job. It would be just like the one you have on your boat. Just have to attach to the batteries and plug in. Most on board chargers have the maintenance abilities now a days. Just have to decide how much you want to spend on one.