A good resource is the US National Forest Service where they will mail you detailed maps of the national forests and adjoining Bureau of Land Management lands. These show public lands where camping fees are half what they are in private RV parks and the public land campgrounds will be a lot more scenic.
In addition to the USFS campgrounds and BLM campgrounds there are state parks and county parks and even city parks with excellent RV facilities and these too will be cheaper than the privately run RV parks. With the US Forest and BLM campgrounds there are ones that are classified as being for "dispersed" camping and the fees are even less and no reservations are accepted so if you arrive by noon you have an excellent chance of finding a space. These dispersed are "primative" so you need a self-contained RV with its own fresh water and toilet.
For California the book by Tom Stienstra is fantastic. Wish people would do comparable books for other parts of the USA but if they have I have not been able to locate them. His book "California Camping" is great for trip planning.