There's one exception to this I would add.  I've wired in a timer circuit
on the fridge so that I can set it to stay on for a fixed amount of time
after engine shutoff when running on DC (adjustable 0-60mins, I usually set
it for 45mins).  This way if you stop for gas or a quick lunch the fridge
doesn't miss a beat, but you'll never run down your spare battery too far.

On Fri, Jun 1, 2012 at 9:39 PM, Dennis Haynes <d23haynes57@hotmail.com>wrote:

> As wired from the factory the fridge only works on 12 volt while the engine
> is running and the alternator is actually working. There is a relay under
> the driver's seat and one of the fuses near the driver's door is for this
> circuit. The fridge is an absorption unit so it is a heater that makes the
> thing work. It draws ~7A/H so it really is not practical to run it off a
> battery with the engine not running.
>
> Dennis
>
>