We get our 6 volt GC2 batteries from Samsclub, first set we purchased at $60 each a decade ago were Interstates and still had life in them going on their 8th year, we traded them in for our current set of Duracells at $80 a piece, they have provided 3 years of service thus far. We do most of our RVing off the power grid and our batteries are worked pretty hard, we use solar and generator to get them back up and ready for another day of heavy demand.
We have an Iota IQ4 converter on our rig and it does a good job of keeping our batteries happy and healthy, we keep our rig plugged in 24/7 when stored in our side yard. I buy them from Samsclub as they have the lowest prices in our area, I would buy the more expensive higher-end batteries but don't see a reason with the longevity we've experienced. Proper charging and routine maintenance is the key, most batteries are produced by a couple of MFGRs anyway.