John, I too have the Duracell AGM 31 series batteries. Two for my 24V trolling motor and one for my main starting battery. It runs my big motor, my bilge, livewell, all my electronics, and my kicker motor.
The price is great and you get an 18month warrenty on them. I have the same setup in my dad's boat with the same batteries.
I've had the cabelas AGM group size 30 batteries and have had mixed results. The ones that were good lasted quite awhile but, you have to make sure you are getting them with the same date code. You can look on the shelf at the store and one of them will be a year older than the others. Of course, it dies quickly too.
I run a ProMariner ProSport 20 plus in both my dad's boat and mine. I like it because it is "distrubuted" load. Meaning that it puts equail amps to each battery until one is charged, it will then take those amps and redistribute them over the rest of the batteries.
I always setup my charger in the back of my boat and run extensions up to my trolling motor batteries. This way I can reach in the back of my boat under the cover, pull out the cable and plug it in. Some boats do not have openings for wiring in their battery compartments. Makes it hard to run that setup.
I have OCD "Obsessive Crappie Disorder"