I had mentioned Solar Battery Tenders, and also should have pointed out positive feedback from users of the Power Tech products to.
12 Volt Solar Battery Chargers & Solar Panels by PulseTech
For between $178-220 you could get the charger only, or for the higher amount the pulse and charge combo. (I see no downside to the 'pulse', and many positive reviews from the boating community on both extending battery life and faster recharges. 'If' the faster recharge is accurate, then paying the small amount extra for the 'pulse' model charger, would maximize your chances of keeping the batteries charged on semi cloudy days.)
But as shared, if AGM, and the battery negative cable is removed, or a cutout switch, the drop is relatively small.