I can confirm that on 6/14/22 I spoke to the camp host at El Capitan and he said we could indeed use the dump station at Carpinteria at no charge and we did. You just drive up the the Carpinteria check-in booth and show them your window tag from El Capitan. You then have to do a u-turn and go over by a large eucalyptus tree where there have 2 dump stations. Carpinteria State Beach is about a 5-minute drive off the 101 through an urban area. It was very simple to do.

Not at this site, as the other answers note, but there is a 76 Station in Goleta not too far away that offers an RV pump out for $7.50.

No. If you are traveling south, you can go to Carpenteria. North to Pismo