"Larry" wrote

There is not a reliable way to charge the batteries together
without using
a battery isolator. Batteries ALWAYS charge at different rates and an
isolator will allow for this.

Bunk. Multibank battery systems were on boats for decades prior to the
marketing of "combiners", "isolators" and other solutions to the
"forgot to switch from Both to House after engine shutdown problem."

Paralleled banks of differing charge states will charge just fine when
hooked up to an alternator. The banks will automatically charge at
different rates until all banks are fully charged, without additional
devices. Consider the 2 bank worst case scenario, a fully charged
bank at 12.7v resting voltage and a dead flat bank @ 10.5v. When the
alternator kicks in the system voltage jumps to 12.5v or so depending
on bank capacity and alternator output. The charged bank is
essentially at system voltage already and accepts little to no
current, while the discharged bank sees a 2v differential and accepts
all the current the alternator can deliver. The charged bank will not
accept charge until the voltage of the discharged bank rises to the
resting voltage of the charged bank, then, as the system voltage
further rises, both batteries charge until the system voltage is
around 14.4v. The charged bank accepts about 2% of its capacity
because it's already charged (its highest charge rate during this
engine run, so it's not even warm), and the discharged bank slowly
accepts less charge until its charge rate is 2% of capacity. Both
batteries are (nearly) fully charged.

Some caveats here. The banks must be electrically similar, mixing
GelGell batteries with regular flooded lead acid batteries is a no-no,
for example. If the banks differ in capacity, the alternator must be
no larger than 30% (GelCell, etc. 50% or even higher) of the smaller
bank's capacity, to prevent damage from too high of a charge rate as
the smaller bank sucks up the current. And, the cabling system must
be properly sized, so all banks see the same voltage; a windlass
battery in the bow with too small wiring will show system voltage when
no charging devices are on line, but a half volt or more drop when the
alternator's pumping 70 amps through it to the discharged battery.