This is what I would do.
Configure a "black box" with relays suitable for switching about 16VDC, and use the 555 Timer IC to switch the relays at a reasonable interval ... I would think 1 hour each would be fine, but even 1 day would work. It's just much easier to use the 555 for shorter periods of time.
Now what is happening is your float charger ( a CTEK brand unit is perfect) will charge each battery for a reasonable amount of time, indefinitely. You avoid issues with paralleling batteries and there is no problem with the charge since even an hour a day would be enough to keep a lead-acid battery in good condition over any reasonable number of months.
If you can solder (or are willing to buy a proper crimp tool) and have the resources to order a few electronic parts, you're golden. If such a project seems daunting to you, get someone you know to do it for you. It's not a complex project for anyone familiar with electronics. There are a number of DIY-type websites you could ask questions at who will help you avoid problems and flesh out the details.
The alternative is to buy a Commercial Float Charger designed to work with a number of batteries ... what it will be is what I've described above, only charging you $600 for the privilege of owning one.