question

Another option would be to parallel up multiple DC-DC chargers. There are also larger DC-DC chargers available like the 3 KW Safiery Scotty which is bidirectional. Sterling also makes larger chargers that are available from Victron.

@Dr P Yes that works. It is what I do. What Smart Alternator are you using. Make sure that it can really handle Lithium and control its output based on alternator temperature. Something like the Wakespeed 500 is an ideal alternator controller and will also interface to the Cerbo GX (VenusOS) for monitoring.

Generally smart alternators are controlled by the vehicle's electronics/computer. Make sure what you buy is compatible.

I'd make as few changes as possible to the vehicle. @Kevin Windrem has given good advice.

Since you would be upgrading the alternator anyway, another approach would be to keep the vehicle's charging system intact and add a second alternator and lithium-compatible controller just for the house battery.

Some thoughts.

As @Bathnm said, it will work.

Need to be careful about switching the Orion. This setup is likely to charge the starter from the house battery when the engine isn't running if the Orion isn't set up properly. Any other loads on the starter battery? You could end up with both batteries being too low to start the engine.

How will you handle starting the engine if the starter battery fails? You have two incompatible chemistries. And the likelihood that the BMS in the lithum will prevent starter from getting enough current.

Any other chsrging? E.g. solar.

@kevgermany you are correct. The ignition switch is wired to the remote on/off. This means that when the engine is off nothing is going from the Lithium house side to the starter battery.