"If you only have a 12 volt charger, you can charge the individual 12 volt batteries one-at-a-time without rewiring anything - your charger's negative terminal should not be connected to "Ground"."
Note 1: The above statement is correct, however, I would strongly advice to disconnect batteries from each other before charging the 12V battery with a 12V charger. This helps with efficiency of charging.
Note 2: Even though a 48V system is normally charged with a 48V charger it is not the efficient way to charge. If you can use individual chargers per battery and able to disconnect batteries form each other before charging this is the best way to properly charge the batteries instead of charging them as a battery bank. But for practical purposes, if the battery bank is charged as a bank then use the manufacturer recommended equalization charging to bring each battery voltage back to nominal voltage at recommended interval. This process will ensure the individual batteries carry the load equally and will reduce internal resistance from developing and building and damaging the bank eventually. Remember it only takes a cell or a single battery to damage the whole battery bank performance.