Suggestions please!

If looking for no-fee camping sites, the forest service uses the term dispersed camping sites. 14 Day maximum stay no fees, no features. If looking for rustic campgrounds, the Army Corps of Engineers have sites in addition to the BLM and Forest Service.

Am not out there now, but can understand your trepidations.
If you can believe them try a couple of the you tubers channels and their spots. A member who shows up in here occasionally has a channel called "Grand Adventure" His security video
He also has one on specifically how he finds boondocking spots. You may find them helpful, maybe not.
Or maybe Love your rv Both will give you some ideas for methods of selection and areas to visit.
Personally, for getting started, it may seem counterintuitive, but go farther away from towns or attractions. Being near a large lake, reservoir, or other tourist draw, also draws un desireables. Go for the middle of nowhere a few times. Then look back towards people.

In Califonria U.S. 395 from the intersection with CA 14 is loaded with dry campsites and boondocking opportunities. Suggest you stay around Lone Pine or south thereof.

If you have specific National Forests or BLM districts go to their website and they will show all their CG. Freecampsites is very good. Or just a good paper map and look for the little green tree symbols that a lot of them use for campsites. I don't have it but a lot seem to like the Allstays app, I think it costs $9. You basically have to do your own research. CG near a city are asking for trouble. In my many years of camping I saw a situation like what you describe once, and it turned out to be a camp out of the Rainbow Tribe (may be extinct now) a mob of wandering hippies and bums. But we do not camp near any large cities.

https://freecampsites.net/ This includes a nice trip planner.