I run a 120W solar and carry a 1kva gennie on long trips. I can also run off a dual battery in the Landy. Its suprising how much power these units chew through especially when set to freeze in warm conditions. I use a semi deep cycle style battery that can still take a quick charge without cooking it.
At a pinch a pair of jumper leads running off your main vehicle battery can pump in a solid shot if your fridge battery is getting low.