I'm equally a bit confused around Lithium charging with the V BMS 12/200.
A lot of the issues of solar and alternator charging are exactly the same in the RV / van culture. Same problems, minus the water. But the solution offered in all the RV installs seems to focus on DC-DC B-to-B chargers.
Here, and in videos around Lithium charging on boats, I see a lot more focus on building something different - adjusting or replacing the alternator regulator controllers, or paralleling banks of both SLA and Lithium as a single hybrid bank, or other solutions. This BMS 12/200 solution (seen used in Journey with Jono vids too) seems to be Victron's newest way. But why aren't I seeing mention of it for RV uses? Only for boating uses. Maybe just hasn't broken into that market yet.
In my "when I have enough money to retire" dreams I would have been planning to use DC-DC B-to-B as the way to charge Lithiums. It seems the simplest thing, as it totally isolates "13.xV Lithium charging" away from "alternators designed for 14.4V SLA". Paralleling multiple small DC-DC even allows for switching one or other charger off to give control over slow charging (on a long day's cruising or nearing the top of the charge) vs fast charging (when needing to run diesel for an hour in winter for a quick boost)
The BMS 12/200 is using a fuse as a simple resistor shunt to measure current flow, and behind the scenes it is somehow adjusting the massive amount of current it is allowing the batteries to pull from the alternator. What logic or magic is happening behind the scenes inside thay BMS 12/200 is all hidden behind the victron curtain of mystery. It's not clear whether it is reducing charge being pushed when near to the top of capacity, or reducing charge demand while the engine is first warming up. But on the other hand, it seems to be Victron's main focus now for A-to-B Lithium charging, so I guess that's a good sign that it will replace the DC-DC converters for that purpose and hopefully get some good victron support. It's not truly A-to-B - as it still needs what they term an 'alternator protect' battery - a sacrificial SLA battery to dump load into whenever it shuts off charging - which I guess still makes it a B-to-B. It can also handle upto 200A, which makes it suitable for large alternators quickly charging large Lithium banks - whereas DC-DC converters will cost a lot more Amp for Amp. A 50A Victron DC-DC is quite an expense compared to a 200A Victron BMS 12/200. But then, once you've committed to Victron charging, I guess you're committed to overpaying for victron batteries. DC-DC solutions will support non-victron banks more easily. There are opportunities to save costs on Lithiums, but perhaps saving costs in one place creates more cost/complexity in others.
I would put some time into finding out more details about the BMS 12/200 before making any decisions on DC-DC converters.