pianotuna wrote:
Since you appear to rarely be "off the grid" I'd go with a single AGM battery. To keep the AGM happy may require replacing the converter.
For somebody who rarely goes off the grid, any battery or a pair of batteries should work if they don't run it flat dead when off-grid. It is still unclear why his flooded 6V failed after 3 years of mostly on-grid use. Useless to guess why, I think.
OP - batteries are all different, you can't blindly trust the converter. Read the battery manual and do what it says.