Take a look at Battery Tender. They even make a solar powered version. I have used their products for years to keep my motorcycle, boat and RV batteries in an optimized state. When I take my Montana to the storage building I always disconnect the batteries and put them on the tender. Here is a link to the website. There is a lot of good information about batteries here. Take a look around on their site:
http://batterytender.com/rv.html