I'm an Ordinary RV Guy, not a Battery Guy, Solar Guy or Generator Guy. But based on my very limited experience with Twin House Batteries, PLUS years of reading this Forum:
1. Two 12V Batteries in Parallel don't play well together. It seems like one injures the other.
2. Even if two 12's are brand new, same make and model, this can persist. So an AGM and a Flooded Cell battery can't possibly be "matched."
(So, 3. Two 6's in series is probably the better idea.)
4. To add to the mention of Start Battery (to be used for your Chassis) vs. House Battery (power you Coach lights, appliances etc.), you said this Northstar battery was rated in CCA. That's usually for Start/Chassis batteries. Coach/House batteries should be Deep Cycle and they are rated in Amp Hours. Start Batteries don't last long in Deep Cycle use.
5. What's the actual distance (the wire run length) between your two House Batteries? You will need very good connecting wire between them. I'm concerned that your Charger might be seeing that the (electrically) closer battery as fully charged and laying down before the more remote battery is.
(So 6. I think the wiring has to be heavy, with good connectors, and all kept very clean and tight.)
