1.  Installing an onboard charger and figured I'd better finally break down and install a circuit breaker for the TM.

http://www.***.com/descpageMGA-MGMRB.html

I assume that is what I'll need.

2. Had always disconnected everything from the batteries when charging, will not really be feasible after installing this charger.  Any precautions I should take when charging?

Yep, that is what you need or one just like it with automatic reset.

Unplug you trolling motor while charging or disconnect one of the motor leads depending on how yours is wired. It is just a safety precaution in case the charger develops a problem so there will be no complete electric circuit to the battery thru the trolling motor.  

When ever you get yours finished you can show me how to do mine.  I need to install a 2 bank charger this year and wanted a circuit breaker as well.

Yep, that is what you need or one just like it with automatic reset.

Unplug you trolling motor while charging or disconnect one of the motor leads depending on how yours is wired. It is just a safety precaution in case the charger develops a problem so there will be no complete electric circuit to the battery thru the trolling motor.

TM is always unplugged unless I'm fishing, so I've got that covered.  Just wondered about everything attached to the other battery.

Everything else should be powered thru an on/off switch either remotely or internal unless you have a Lowrance GPS unit and haven't installed a diaconnect switch on the antenna power cable. I don't know of anyone that disconnects the accessory/starting battery when charging.

edited: another note, you will have to make an adapter for the circuit breaker connection to the battery or purchase one made for that purpose. It is just a piece of flat metal with a small hole in one end to fit the breaker bolt and a larger hole in the other end to fit the battery post. Use copper if you can. You'll need to adapt the motor cable to fit the other side of the breaker too. The breaker is about the size of your thumb's second joint.

Wow that's cheap!  Minnkota wants $45 for theirs.  But then it's a 60A breaker.

What kind of TM you got? My Minn Kota explicitly recommended their 60 amp circuit breaker. Which is what I got. Figure if it dies they cannot argue that point on the warranty.