A few years ago I replaced our last pair of messed up 12V batteries with two 6V. I bought a pair of Duracell 6V AGMs at Sam's Club. So far so good.
It was the second time replacing our 12Vs because one of the two does more work and fails sooner than the other. Then comes the acid boil-overs. I decided to try a pair of 6Vs in series with hope they will share the load properly and therefore perform better as a pair and live longer too. The same theory with batteries in a flashlight, stacked up inside, all being drained at about the same rate.
If changing over from 12V to 6V, do your research on the difference in height. They need to fit in the same compartment. I removed the battery tray in our rig to gain the height. The no-maintenance sealed AGMs means no need for a tray anyway.