Thumper Pack
I bought one of these over 10 years ago and use it as a mobile dual battery system. Plug my fridge and camp lighting into it and then plug the thumper into the vehicles 12v plug. That way when the ignition is off, the fridge etc draw off the battery pack and everything charges up when the vehicle is run again.
Being partable you can lift it out to put in tents, picnic etc but also move it to another vehicle (boat) etc if required. Also perfect for jumpstarting others vehicles, you dont have to hook up vehicle to vehicle jumper leads and ever risk spiking anyones ECM etc.
Worked so well I have never installed a dual battery system in any of my 4wds yet despite owning two systems in the the shed, DBS and Cetek dc-dc.
Matt.
ps. you can make your own version of this with the supercheap alternative...
Ark Battery Box and throw a sealed VRLA or AGM battery in there!