I did a lot of research on drop-in  LiFePo4 batteries (the type to use) and finally settled on the 100 ah "Battle Born Battery". Both "Bay Marine Supply" and "Northern Az Wind and solar" sell it and both currently offer a discount over the US-based mfr's price,  though you do have to ask Bay Marine for the discount.  Bay Marine is a GREAT company to deal with, by the way, very helpful and the best prices around. Nothing against NAWS either, good folks. 

100 AH might not sound like any great advantage considering the high initial cost ($850 shipped with discount in 2017), but these lithiums should give much more real usage for the AH rating, size and weight.  

- you can draw lifepo4 batteries down all the way if you want with little issue, so 100 ah lifepo4 has more useable power than 200 ah lead.  Most sources indicate that 100% DOD is not an issue when needed, but that 80% DOD will give the most cycles - they hold their voltage up until the very end, so your fridge won't be cutting out early. - they charge much faster to 100% and will soak up more amps when available, so smaller panels or shorter drives will still get you your full capacity back sooner.  You don't have to "spoon-feed" them juice over long periods like the AGMs and other lead based batteries. - unlike lead, they don't mind if they don't get fully charged, they actually last longer. - minimal self-discharge -longer life in storage -many more cycles

First, I ran 4ga from front battery as suggested.  Prepped with padding, custom Simpson tie and duffle strap battery hold-down.

Basic wiring
Battery hold down
Close up modules on plywood

Next, I had to work out the mount for the battery which ca n't make use of the original mounting system.     

Next, organize and wire in the components.     

        I managed to get everything into that one compartment and mostly serviceable from front access panel.     

        Very pleased with Victron components.  Solar charge controller (MPPT 75/15)  and Battery/system monitor (BMV702) both have bluetooth so I can monitor by app on my phone.            

View of module access front of seat

The plywood mounting panel also acts to as a battery stop. It drops in from the top and is wedged against the battery with triangle blocks on the ends of the seat base that are then locked in with a single screw. To loosen battery or work on wiring, remove screws and pull wedges.

Completed install

Connections Include:  

  • 4 ga wires (should have been thicker) from 80 amp breaker at main battery  
  • a 12V DC marine grade outlet in the back where the Hella was.  
  • A DC power jack in the same area for charging with a portable 14v charger plugged into AC on rare occasions  
  • 2 heavy gauge wires going to passenger seat to power Victron Pure Sine wave inverter.  
  • BM-702 monitor gauge on side of seat as seen.  
  • Anderson jack mounted for plugging in portable solar panels. Chose to not install panels permanently on roof as we like to park in the shade and only use solar when camping longer periods.