Engel battery

ozidave,

You could simply use your car battery and run the vehicle (or go for a drive) after the second day. This should keep the battery charged.

BUT, as a backup I would strongly recommend a portable "jump starter" pack that you could use to jump start the vehicle, should the car battery drain down too much. These are available from Repco, Supercheap, etc. for a reasonable price.
Buy the highest capacity unit you can aford. (eg. 400 or 500 amp unit)

These are also useful for powering lights, etc. when camping, so are a worthwhile investment. They simply charge from the vehicle cigarette lighter socket whilst travelling and utilize a non-spill AGM type internal battery.

you can buy a low power cut off device for around $20. that plugs into your cigarette socket and the engel plugs into the other end and when the volts gets down to a safe engine stating level it will cut of the appliance,when you see this happen just run the engine for a while

i have seen them at our local camping store in adelaide (snowys) but places like battery world and the 12volt shop should stock them,i once saw it in k-mart but haven't seen since,shop around prices may vary.it looks like your 12v mobile phone charger on sterroids i think it has a couple of l.e.d.s too

A $99.00 GMC generator and battery charger. Run fridge from 240v and charge battery at same time.