We use the same big tome that Norm, Jon and Mike have mentioned. You can get it at any Camping World. If we have internet access and I know where we're heading I'll also check out Reviews of RV Parks and Campgrounds - RV Park Reviews
. The reviews can be useful, but you have to read them with an eye towards what you're looking for. What one reviewer gushes over may not be at all what would appeal to you. We also like to stop at a good visitor's center when we get to a new area. They often have good maps and state and county park guides, as well as knowledgeable personnel. We also carry paper road atlases for many western states published by Benchmark Maps: Benchmark Maps & Atlases
. I think they only make these for the western states, but they're great resources not only for campgrounds, but for all sorts of other recreational stuff. The atlas you show on the right in your picture looks very similar to these atlases.