There is a site Free Campgrounds.com that has sites that are $10 and under listed. Worth a visit.
A few books that are helpful are:
The Ntional Park Service Camping Guide
RV Camping in State Parks
also two we like and use a lot are
Rest Areas & Welcome Centers
The Next Exit (use it when moving on interstates)
We have been at this for 3 & 1/2 years and also look a lot of stuff up on the internet when we are parked. Also some libraries have good info.