I agree with MikeMl's solution. One other point is you say in normal operation the batteries ate in parallel. I would have thought it better to have one for engine starting and another for all other loads. With the batteries in parallel if the pair become discharged then how will you start the engine to recharge them ?
Les.
In my Cabin Cruiser, both batteries are flooded-cell, lead-acid; one is a deep-discharge house battery (for running cabin lights, water pump, the stereo), the other is a starting battery. In a pinch, the house battery will crank the V8, but the starting battery has higher CCA.
Normal operation: both
off when on the trailer.
2 for starting.
1 for camping.
Both when cruising. Since both batteries are similar in chemistry, the alternator voltage regulator holds the selected battery (either or both) at ~14.4V during recharging.
My boat sits on its trailer (under roof) for months at a time with no AC line voltage available. I use a solar panel charger to "float" the batteries independently.
To enable periodic "equalization" of each battery, I use the alternator during cruising. I tricked the alternator controller to temporarily raise the charging voltage (using a toggle switch) to ~15.2V, which is sufficient to cause the cells in the selected battery to outgas as recommended for equalization of the cells.
The sense-wire for the alternator controller samples the voltage at the common point between the battery selector switch as I indicated. The voltage selection switch (normal charging or equalization) was put inside the feed-back control loop inside the voltage regulator. Now there are two trimpots that set the output voltage.