Dave,
sorry to say that my post was in preparation for doing this work which has since been delayed till next month, so i'm not sure how much i can help at this point.
What we decided to do was based upon only the passenger seat base being available for pwer systems, as i have have had subwoofer and amp fitted under driver seat base. We also have not done the full camper conversion but kept things as a day van ie no cabinetry within which to hide additional batteries and consumer units etc.
We have therefore bought a big lithium battery which has the capacity of two smallish AGM batteries, and which is the largest that will fit under the passenger seat base. We think we will be able to fit most of the associated systms under the same base, and run the necessary feeds out to the enging bay, and upto the solar panel on the roof etc.
I'm sure you will find other threads giving details from those who have completed their work on this forum, so these guys will actually have knowledge to share
sorry can't be more help yet,
good luck!