Still don't have enough to go on to say you need system x or system y.
The truck has a convenient on-board battery charger; the alternator. Let's guess that it's a 105 amp alternator. I'll take a SWAG and say that at 2000 RPM the alternator puts out about 30 amps. Your total load on the battery must exceed that for the battery to begin to drain. That's 360 watts more or less.
If all the stuff you are running is more than that you need to either split the load between two battery banks or get a bigger alternator.
If you want a spare battery for emergency starting, just keep one in the back and swap it out when you need it. Don't even bother trying to cable it up.
If you want to run your stereo and fridge and winch and not compromise the start battery you need a dual charge system.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
All my troubles are Rover