Why would you mention that the battery charger in your inverter is ON while driving? When you are driving it is your alternator that is charging your batteries unless you are running the generator as you drive. I have a Magnum MS2012 and the ARC50 remote, so yes, the charger switch is on all the time, but it’s not charging the batteries as I drive unless the generator is running.
Since you just completed a battery change I would look at the wiring of your battery bank, the connections to the alternator, the connections between the battery bank and the inverter, etc. And of course the voltage of each battery in the bank. One bad battery can bring down the entire bank.
There is also a chance that when you disconnected the old batteries that you did not follow Magnum’s explicit instructions to always disconnect the positive connections to the inverter first. Disconnecting the negative connection first can damage the inverter control board. Many inverter installs include a rotary cutoff between the inverter and the battery bank to facilitate this requirement, but the cutoff isn’t always evident.
When I’ve contacted Magnum they’ve always responded to emails quickly. But only on weekdays.
Keep us informed about what you find.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad