Question on trickle charging 2 marine deep cycle batteries

[bonginator]

We just removed our 2 12 volt batteries from our AS and also have purchased a battery minder trickle charger.

The batteries are now in the garage and wired in parallel. I notice that the instructions seem to say to attach a 24" 6 guage AWG wire to the negative post before connecting and charging with the trickler. Is this required to "ground" the batteries while out of the AS?

Also - it says something about refilling the batteries with water but I don't see any water line or water inside the marine batteries. Am I just missing it? Each battery does have 2 caps on top with 2 holes inside each cap.

Thanks.

Sandy