Before covid I used to find good parking around L.A.
Echo Park was always a good spot to post up in for a few days, as long as your van is within the height/length restrictions. The park has a few homebums in it but they were all pretty chill. Plus the bathrooms got cleaned every morning so if you go in early they'll be both clean and well stocked with TP.
I would also find good parking near Venice Beach on Washington Blvd. If you don't mind a walk to the beach/bathroom there is always parking around the Abbott Kinney / Washington Blvd area.
The Arts District in DTLA also had a few good spots but offered not much else other than a place to park. The area north of 7th between Santa Fe and Alameda is both quiet and doesn't have any height/ length restrictions so you could park an RV or Bus there with very little issue.
Again, this was all before Covid. Last time I was in LA was October of last year so i imagine not too much has changed.
Good luck!