Best quality battery? Can I use 8 batteries?

I have 6 Deka AGM batteries. The cranking amp of these batteries isn't the highest, but they can stand up to the discharge-charge cycling much better than conventional wet cell batteries. My Volvo battery compartment is roomy enough to stuff two extra batteries, so I did.

Anyone put a more beefy alternator in their Thermoking APU. The puny one I have can't keep up with my cooking amp needs very well.