One of the most important things a "modern" battery charger can do is change its charge profile.
For plugged in long term storage 12.8 to 13.2 is as high a float voltage that should be set.
When cruising the normal higher set points will result in a faster charge.
An older batt going bad is not uncommon , sadly it overcharges itself , and usually boils out the second battery as well.
Its really low tech but a Battery Tender on each battery is one solution.
have no idea weather the cheapo China stuff works , but these do.