How you plan to use your Dual battery will directly effect the design
The addition of a second battery is seeking to either1) Increase starting power 2) Establish a separate House power supply to your starting battery3) Or increase your starting batteries starting and house power capability
Aim of Dual Battery
More Engine Starting Power
By adding that second Battery you can double the amount of Battery Cold Cranking Amps or CCA making that larger motor start easiler
Protected Separate House Power Supply
By creating a separate stand alone 2nd battery for house power you can run the fridge and or accessories without any chance of flattening your starting battery essentually having 2 separate power supplies
Increase Both Starting & House Power
By having both batteries wired to work together you can increase your CCA and house power capacity but without any protection that you can flatten your starting ability by over use of power
Once we understand how you plan to utilise your 2nd battery we can then look at what size battery will meet your needs.
Then where it can fit in your vehicle, the choice of isolators is important decision and also the typres and quantity of plugs and locations of them.
Depending on if your running a single 30 or 150 Litre fridge or multiple fridges or camplights, winches, in car tv's and accessories will depend on the amount of power you will use.
Supplimentary solar power and interlinking with your caravan power supply may influence how long you can run these items without the help of your vehicle alternate re-charging the batteries. We need to understand if you plan to stay off grid or if your looking for power for several days.
