You have gotten excellent tips on spots to overnight. We have a tip on where we never overnight and that is in highway rest stops. Particularly in the East. Of course urban blighted areas should be avoided.
We have mostly boondocked but we have enjoyed many of the National Parks, State Parks etc. The few times I made reservations, ie. Yellowstone and Denali, we wound up regretting that we had reserved too many days. Three days were enough, not a week. Camping in Yosemite Valley is not to be missed, I believe. We were able to get a cancelled spot in Yosemite by lining up early at the reservation building. In Maine we found little County Parks, waterside, for no fee, and stayed for several days. We have been steered to good locales by folks in tackle shops, hardware stores etc. There is no substitute for local knowledge. Ask...........
Be bold, be careful, low profile, have fun!