1. You could add the house battery under the hood with a custom mount, but it will be tight. then you will have to run all of your wiring INTO the cabin to run your loads. And then separate the two electrical systems so you don't accidently kill your starter battery. IMO, just run your house battery inside the van itself....
2. You don't want to use your starter battery to power your loads anyhow.... so the AH's are enough for the van, and just leave it at that....
3. Depends on the house battery type...
traditional lead battery (AGM, deep cycle, lead acid, etc) - YES. These won't demand a huge charge current from your alternator when they are running low.
Lithium? NO. They can kill your alternator at low RPMS since they will demand a huge amount of power when depleted. You will want to use a lithium specific battery relay (such as the LI-BIM 225), or use a DC to DC charger (aka battery to battery/B2B charger) that acts as a current limiting device, and gives you the proper charging profile for lithium.