I had looked into various battery strategies (single, dual), and considering my use case, I have decided to start with a single large capacity, deep cycle battery. As it is much simpler, easier, and lower cost to implement than a full-on dual batteries with isolator setup is.

My use case:
- weekend and short(few days) offroading trips
- driving around during the days, and camping during the nights
- maximum "offline" (car's engine not ON) time will be no longer than 36hours
- powering (offline) ARB 50QT fridge
- powering (offline) HAM mobile radio
- powering (offline) some low power LED light strips

Like many others on this forum, I have decided to use a 31M-PC2150 marine grade, deep cycle battery, from Odyssey (some claimed that Sear's Diehard Platinum 31M is the same thing).
Amazon.com: Odyssey 31M-PC2150ST-M TROLLING Thunder Marine Dual Purpose Battery: Automotive

SPECIFICATIONS
Pulse (5-second) Hot Cranking Amps (PHCA): 2150
Cold Cranking Amps (CCA): 1150
20Hr Nominal Capacity (Ah): 100
Reserve Capacity Minutes: 205
Dimensions L x W x H (in): 13.00 X 6.80 X 9.41
Weight (lbs): 77.8

Some useful info about this particular battery (voltage vs % of charge):


This battery comes with brass SAE posts, 3/8 inch positive and 5/16 inch negative thread SS stud terminals. And in order to it to hook up to the "factory" clamps without ANY modifications, ordering some Remy Battery "Stud to Post" converter is THE WAY TO GO! (Be sure to order 1 of each size) Stud to Post Conversion Adapters - Remy Battery

Also, while you are ordering from Remy, I found that their group 31 battery to be the lowest cost solution ($8.44!!!) to house the battery (a $100 battery cage is NOT needed!!): Steel Battery Tray (13") - Remy Battery

Post converters screwed in (and extra converters) and the battery tray:


Size comparison between the Odyssey 31M vs the stock Panasonic:


Once you removed the factory battery and the whimpy plastic tray, the new steel tray fits right in, and you can get a "M6 bolt/screw" (maybe 15mm long) and a washer to screw into a threaded hole as you can see in the picture below. Together with an aligned "hook", this tray is not going anywhere!



Unfortunately, with the added battery try, the factory "J-hooks" are not long enough to hold-down the battery, you would need some 10in hooks. You can get them from Amazon cheaply (with prime too): http://amzn.to/1PHfrFm

I am not a big fan of the "wing nut", hence, I dropped into home depot and picked up some 1/4" (IIRC) nut/washer/lock-washer for the "J-hooks". And 1 last final touch (not really necessary), is to use a bolt-cutter and trim off the extra length of the J-hook that is closer to the fuse box).

Here it is, all mounted!


SOLID and economical!!