Originally Posted by boywonder
Thankfully that's not the case or all of us are wasting $$ on big house batteries. When either the alternator is spinning or the solar is solar-ing my large house batteries and my measly starting battery are connected in parallel by the separator and they all manage to get fully charged.
But on the other hand, your van battery and your house battery are likely separated by either an isolator or a separator. I think the OP's question is about his two van (starting) batteries being different sizes/capacities, which IMO is a whole 'nother animal. I don't think batteries that are directly tied together should be different, but that's more personal preference than anything I've personally researched.
Recommend getting Scalf77 to weigh in on this. He's the van electrical guru.