Two 12 volt batteries have twice as many holes to watch for low electrolyte as two 6 volt batteries.
That makes twice as much work.
One cautionary note: Unless the batteries are exactly matched one may charge slightly different, not a problem with parallel connection, but can be with series connection if not "Equalized" and may cause problems over time. RV converters don't normally have an "Equalize" stage. I pull mine every winter and charge them individually with a 6 volt charger to avoid any problem.