I used a small portable charger for my cranking battery. Most of the portables offer the same features as an onboard these days as far as maintaining and charging.

My .02 worth, or this information and a buck will get you a cup of coffee in some places.
Do not get AGMs for the trolling motor. AGMs usually have a lower RC than lead acids of the same group size. RC is the ability the battery has to hold a charge over a given period. The AGMs will make you think they're holding a charge because they are made to deliver the power throughout their discharge cycle. Problem is, when they die, without warning, they're dead and have to have another battery (slave battery) to jump in parallel before they can be recharged (once they're discharged). Also, you have to make sure your charger is AGM compatable or it will burn the battery while trying to charge it. Now, for the cranking, a 31 series AGM is good. This is because it does deliver the power througout the discharge cycle, but with pumps, electronics, and outboard you shouldn't discharge it to the point of a trolling motor battery. The alternator of the outboard will keep it up while you're running, and will just need a "top off" needed when you connect the charger.

Agms will let you know they are dropping in charge throughout the day. They will not die without warning. Lithium batteries will die without warning because they are designed to work at full capacity until dead.

You don't have to buy a 3 bank if you already have a 2 bank. You can buy a single bank and add also. I now have 4 batteries and I had a two back so I just bought a used 2 bank and run them both. I just use Walmart and interstate batteries with no problems. Battery size mattress and a digital trolling motor really helps. I could go two days without charging with a 80# tm if I needed to.

A 70-80 won't be that noticeable but the digital maximizer will.