Onboard Charger Wiring

I have a Guest 2613, three-bank, onboard charger. I used to have a 12V
trolling motor, so I had one set of leads going to the cranking
battery and ganged up the other two pairs of leads to the single
trolling battery.

I was lucky enough to get a 24V Minn Kota Maxxum 65 for Christmas, so
I have been busy reconfiguring the battery/gas compartment to
accommodate the extra battery, and have changed over the
plugs/receptacles and fuse to 24V.

So now, I've got two new trolling batteries hooked up in series, and a
separate cranking battery.

So, how do I connect the battery charger to the 2x12V bank, given that
I have two pairs of leads to work with.

I was in Boat US today, buying some stuff to do the upgrade, and I
asked the salesperson how to wire it. Here is what he said:

"Connect both positive leads to the positive terminal on Battery 1.
Connect both negative leads to the boat's Ground."

I commented that there is no "ground" per se in my electrical system,
because no negative terminals are connected to the (aluminum) hull,
the negative leads go straight to the accessories.

When I told him this, he said that the two negative leads could go to
a negative pole on one of the batteries. He wasn't terribly clear on
*which* battery, though.

Can anyone give me a definitive answer?

One more question: The reason I bought a three bank charger was
because I thought you needed a set of leads per battery, and I knew
you needed three batteries in a boat that had a 24V trolling motor.
Was I wrong in that assumption? If so, there isn't really ever a need
for a three bank charger in a bass boat, is there?

Thanks a lot!
Jamie Jackson
Falls Church, VA