Can I just add 2 batteries?

I have a 2005 Mobile Suites with two 6 volt batteries. I want to begin dry camping alternate weeks. I have a Honda 2000 generator that I use for Walmart stops. Where do I start electrical system improvements? I am currently replacing internal lighting with LEDs. I feel my next step is to add 2 batteries to my existing two 6 volt Interstate batteries. What else would I need and what is the estimated cost. Thanks