Out boat uses twin 12v batteries but has one engine with one charging output. The boat has a selector switch for battery1, battery2, both or Off.
If you select battery1 or 2 then that battery will be charged but the other one won’t unless you use the Both setting, but then you run the risk of one battery discharging into the other and you end up with two dead batteries.
I know there is a good solution from a Canadian company called Blue Sea which is an automatic charge relay that will send charging current from the engine to both batteries when it’s over a predefined voltage (about 13.3v), but will automatically open the relay and isolate the batteries when the voltage drops (i.e. when the engine is stopped).
This allows you to have one battery for engine starting and ‘house’ battery that runs the VHF, stereo, amplifier, chart plotter, sonar, etc.
The only issue is it’s pretty damn expensive (along with anything with ‘marine’ in the name) so I was wondering if the motorhome dudes use something similar because I’d be surprised if you run everything off just one battery.