That is a complete myth.
There is NO advantage to using two 12V vs on pair of 6V..
All it takes is one cell to go bad in ONE battery and it will draw your "good" battery down and if not found quick enough it WILL dagame the "good" battery.
If you have one cell go bad in one of the 6V batteries you will not be running 6V, no it will be the SAME as if you had a 12V battery with one bad cell (about 11V). This will NOT interrupt your trip since most lighting will operate at lower voltages, your furnace may not like it but it won't ruin your trip any worse than finding a bad 12V battery.
It is not a complete myth and there is an advantage.
If I loss one of my 12 volt batteries for what ever reason I can disconnect the bad one and still have a fully functioning 12 volt system. I understand I have half the capacity, but I still have a 12 volt system and everything in my trailer will work. I have had this happen. That is the advantage.