As far as batteries, true deep cycle 6 volts are what you want. Nearly all "deep cycle" 12 volt batteries are more of a hybrid. When you pick up a true deep cycle battery you'll feel the difference. As others have said, you should also change over to a multi-stage converter (Progressive Dynamics) and your batteries will last longer. You also need to know how much you're draining them. Draining them too low will dramatically shorten their life. Check the specs of the battery for that data. You need an accurate voltmeter hooked up to them to know how for down you've drawn them.
I went really hard core and put Lifeline AGMs in mine. Big bucks but worth it to me.