Your Onan generates 12 V @ 1A during operation to prevent discharge of the battery used to start it. It's true you cannot use this output to charge all your batteries, only the one used to start it.
Just curious, don't you have a converter to generate 12 V when you're connected to shore power? When your generator runs, why isn't your converter charging the four other batteries?
If you don't have a converter or shore power inlet, you can always add a Battery Tender or like charger to the output of your Onan to be able to charge your four batteries.
Furthermore, you can add a second Battery Tender to also charge the single battery used to start the generator.
2015 30' Classic, "Chez Nu"
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