I've watched several videos from Australian Direct. They have very informative videos on vehicle/trailer electrical systems.
I contacted them to see if they ship to USA but they do not.
Here is a good video on maintaining and charging AGM batteries:
https://youtu.be/Z3xKYDpGwxs
Here is a comprehensive, dual battery guide (over 1 hour)
https://youtu.be/Z9c1hVXBggk
Made by www.australiandirect.com.au
John