Mobile Home battery charger transformer
As an alternative to Andy's answer, it is also possible that this circuit arrangement is used:



This uses only 2 diodes but needs a transformer with a "centertap" output (the middle connection, which is grounded). You can recognize that centertap by the fact that it has two wires connected to it instead of one like the other two pins.

I think this is more likely what is used as a symmetrical supply isn't needed for 12 V battery chargers.