Hyperion Water Reclamation Plant

They don't publicize it, but RVs can dump for free at the water treatment plant. It's across the street from Dockweiler Beach, where people pay to dump. They are only open for dumping certain days so be sure to call for the schedule. I posted the one I have from 2020.
Enter from Imperial Highway, the first entrance after turning east from Vista Del Mar.
Sometimes there is a line, but regular campground dumps also get lines. You will need a mask and may have to give your name and city.

Current hours as of 2/21/2021:
Monday 9-1
Tuesday noon-3
Wednesday 8-11 and noon-1
Thursday noon-3
Friday 9-1
Closed Saturday, Sunday and L. A. City holidays

Confirming that as of Mar 2022 the hours of operation are still what they are in the photos above. I had to show my ID at the gate so I believe you need to be an LA County resident. And a guy came and asked my name, where I lived and how many gallons I was dumping.