how's the water level in that new battery?
If your converter or charger is malfunctioning and overcharging the battery, it can boil it dry fairly quickly. If this happens, it can completely kill a battery, even a new one.
Alternatively, perhaps you have something turned on that draws a large current and just drains the battery quickly. Perhaps that something is something you have actually turned off, but for some reason does not obey the "off" instruction? You might try disconnecting the battery by removing one of the cables to see if the battery still won't hold a charge. If it doesn't, it is internally shorted out, and needs to be replaced, no matter how "new" it is. Then would begin the hunt for what killed it so quickly.
Good luck.