How do I run my lap top of 12volt RV batteries?

I have two deep cycle batteries in parallel. I am not sure if it is possible, but it would be nice to work on the lap top while I am out on the roads. I tried buying a converter you can plug in the lighter and the laptop works when the car is running but when I shut it off it will stop supplying power. I didn't like that idea anyway Id rather run it off the two deep cycle batteries instead of the car battery (for obvious reasons hehe). Thank you