When driving the alternator charges the van batteries, then the house batteries. When on shore power, the opposite. How many starter or house batteries you have is irrelevant to the charging. That's all assuming you have a separator (post ~05) not an isolater (pre ~06).
Multiple house batteries just give you more capacity. 1 "4D" AGM is approximately (very roughly) equivalent to 2 of the smaller house batteries. Myself I have 2x "4D" AGMs.
What you need to do is figure out how much power you need, how much time you'll spend without driving or being plugged in, and then figure out how much battery you need. Once that's done you can decide whether to supply that need with single or multiple house batteries and how many.
Clear as mud eh?
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it was good to be back
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