He mentioned the stronger and weaker battery, If both are new, and same size, make, model, etc. Then yes, when he FIRST hooks them up one will have a higher state of charge,, This will not last long.
Soon as he either charges them to full, or starts to use, they will lock together and both will have the exact same state of charge.
Balancing... Is done like this
. . . . . Battery---RV
IF you do NOT have a BIG (comma in the watts column) inverter this is not really necessary... I do, so I do, but at less than 10 amps max load with 4GA wire,,, Do not worry about it.
All the warnings about "The higher charged/stronger battery will signal the charger it's full and the other one will never reach full" simply do not apply because that lock step point will be reached first so there never will be a higher state of charged battery.
IT DOES, however, apply to batteries in series.
Home is where I park it!