How is the solenoid for the house/starter battery controlled?
There are a number of systems that sense both battery voltages and will automatically close the solenoid when it detects a charging voltage on either end. That's what I use (specifically, this system:
https://images.carid.com/t-max/items/pdf/47-3800-installation-manual.pdf)
So when the alternator is running, it senses charging voltage from the chassis side and connects the house battery. Similarly, when I'm on shore power or solar (which connect to the house battery) then the system sees the charging voltage from the other end and also closes the solenoid. I have manual breakers at both ends of the house-chassis link so I can sever the connection if needed. The T-Max controller also lets me manually close the solenoid with a button in case I need to "self jump" or want to have extra juice for running a heavy load. In practice, this means that in my driveway, the solar panel on the van ends up doing maintenance charge on both batteries throughout the day. I very quickly got used to hearing the "clack" on the contactor opening and closing as the voltages go up/down.
The downside of this is that the house and starter batteries need to be "compatible" from a charge voltage standpoint. I couldn't run a LiFePo4 house battery with this setup as it would directly connect the alternator whenever the engine was running. It would be easy to insert a DC-DC charger into the mix but that doesn't do the same job of feeding charge back to the starter battery, and I'd lose the self-jump capability unless I added a provision for it.
