I would have the solar (eventually) and the Onan charge the house battery. You're likely to be using the house battery when running the Onan anyway.
Power your cheapo inverter, the lights, etc off the house battery. Possibly make the Onan start off the house battery.
The alternator still charges the van battery.
So now you've essentially got separate systems- the house side with it's charging sources, the starting side with it's charging sources.
Between (my opinion) you want a separator. Normally the separator will be open, leaving the systems disconnected. During charging, from either the alternator or house side (via Onan) when the battery reaches a certain level the separator closes, connecting the other side. The charge continues in the other set of batteries as well.
That way when you run the Onan the house battery is charged first, then the starter battery topped off.
The separator can also be wired with an override switch. SMB has it wired to key-start, so whenever you crank the batteries are connected. Personally I prefer an override switch so I can force them together or separate, or leave in "auto" where the separator will close at a voltage.
SMB uses a PowerSure separator, I upgraded to a BlueSea 400a
. There are probably tons of makes and models.
The brain from the generator and alternator alternatively should keep the system from overcharging, but I'm not sure if there is anything more you need to do than the obvious connection of + and - battery cables to each side of the separator.