Ron, did you use true deep cycle 12V batteries or the combination Starting/Marine 12V batteries?
So far after many years, I've had good luck with twin 12V true deep cycle batteries cabled together in balanced configuration. But they have been AGM deep cycle batteries, so their good performance may have been due to the fact that they were AGM 12V deep cycle batteries advertised as being for deep cycle service. True 12V deep cycle liquid acid batteries designed for deep cycle service are hard to find, so most who have tried 12V "deep cycle" liquid acid batteries and been disappointed may have had only the commonly available Starting/Marine liquid acid 12V batteries.
We have to be careful to compare apples to apples when comparing 6V to 12V.