We carry;
#1 Passport America, although we rarely get to use them, they have at least paid for the membership many times over.
#2 Trailer life guide, I buy one new every year and belong to Good Sam club. I have found their ratings to be realistic and good for finding that nice clean bathroom every few nights to take a shower in a clean place.
#3 We have guide to Military campgrounds which I renew every 5 years or so as things don't change all that much over time there.
#4 last but not least we use an old book of FREE campgrounds I have owned for years. Not too useful now, but I can't seem to work up the courrage to pitch it.
There you have it, somewhere in there We always find somewhere to stay. Yes, State Parks and Corps of Engineers will help if they can, National Park service is pretty inflexible due to Federal Regulations. Most commercial campgrounds will let you dry-camp for 5 to $10. If they want to much, I just say thanks and move on, although, that has rarely happened that we even had to ask the question. As I said, I rarely make reservations in advance because we move around too much and like to experience the "Freedom of the Road" Hope that helps.