There are campground guides that describe and rate the facilities. (See AAA or CAA).  You will develop preferences as you go.  Check the aerial maps to see the campground.
Prices are usually in line with the quality (but not always). Scenic features go down when you get close to big cities. 
Check the facilities available. You may not always need full hookup -- while in transit you can go a few days without a dump. We spent a 2-week stretch in a site with no sewer, but they did have honeywagon service (and a dump station). 
Decide if you need a pull-through or can stand a back-in.