The LR3 is already "prepped" for a dual battery setup from the factory. Depending on your vehicle (left hand drive or right) you will have an extra space specifically for a battery. A number of standard size deep cycle batteries (or normal) will fit.
The set up that I am putting in will have an automated isolater/combiner between the AGM deep cycle and the LR3 battery. It will also have a power run to the rear cargo area. In that area I will have a new 12v outlet and then a quick connect/disconnect plug as well. The deep cycle battery will provide live power to the rear at all times. (Something the LR3 is missing from the factory)
The secondary benefit of this is of course, it is now almost impossible to **** both batteries and not be able to start the engine. We do a lot of camping, hiking, etc., which usually entails hanging out with the truck in some sort of discharge state.
The primary benefit will be the high-current power available at the rear of the truck. This will be up to 30a or 50a depending on the isolater. This will power a portable refrigerator. It is not an Engel, as those are not "portable." The portable fridge I have is a Dometic just like in most RVs. It can run off 120v, 12v, or propane gas. It is about the size of a large cooler. The key thing is that with no ice required, it can hold a ton of critical survival items like ice cold beer, perfectly maintained steaks. . .oh, and other things like milk for the kids. . . I run the fridge on gas (standard camping propane canisters) during the day when I can have the windows cracked, and 12v at night when it is all closed up. The gas lasts for ever. . .the 12v not so much.
In addition to the permament deep cycle under the hood, I'll also have a matching deep cycle AGM in the cargo area as needed only. With the quick connect/disconnect, this will become part of the overal circuit and double dischage time. Having the 30a/50a connection will also allow periodic charging of both batteries direclty from the alternator. (However, most of the time the cargo batt will be charged from a normal batt charger)
Of note. . .a fantastic AGM deep cycle battery that I found, that also fits in the factory location under the hood, is the Sears Platinum AGM, PM-2 34M, $199. This is actually an Oddyssey battery, one of the best you can get. It is also reportedly better than the spirals. It has a 3yr free replacement warranty on it. Odyessy has confirmed that it is their battery, just a different case. The Odyssey equivilent is about $100 more. I am not fan of Sears at all, but this battery is awesome.