I replaced my 2 x 8D with 4 auto batteries (series-parallel arrangement)
I dont have OTR air. It seems to work well (for 3 years now) starting the coach both summer and winter.
They were a matched set originally, but one of the batteries failed internally and drained all the batteries,so theres one oddball.
My only other suggestion would be when paralleling the 2 batteries is that there be a provision for isolating each of the batteries from its mate (in case of failure,etc).
It works for me!
Mark