Not sure what part of California you're in, but a lot of rest stops don't have them. I'd say I know of more that don't have them than do have them. Last time I was in Oregon and paying attention, I did see that a lot of their rest stops did have them.
There is a book you can get a Camping World that is a directory of RV dump stations. I never bought one because I never had a problem finding a place.
2021 Nexus Viper 27V. Class B+
2019 Ford Ranger 4x4