Excellent context, thank you.
I have a travel trailer with 50 amp service. Picked it up in the fall so the system is your traditional OEM to get it off the lot, no inverter and a single deep cycle marine battery. If you're managing with 440ah of lead acid I feel anecdotally I'll be fine. My current plan is Multiplus 12/3000, 150/60, 800 watts of solar with room to move to 1200 watts and batteries ... well that's why we're here
.
I've been looking into the larger BMS's, the brand you mention pops up often. It's pricey but that isn't the reason for my hesitation, I am concerned about the risk and complexity of introducing an external relay. I haven't spent enough time researching them to understand how they handle failure, fail closed as an example. Curious, you're designing for a single bms to manage two batteries? Didn't know that was possible.