We've found that these days travel is down and reservations aren't generally needed. When we do want to make a reservation for the same day and they want a CC we ask "how full are you?". We prefer to not give them the CC information for CC security and to allow us to change is something else comes up.
Be advised that popular coast CGs will likely be full and require reservations perhaps months in advance. We've had good success with OR state parks reservations a few days in advance. OR state parks are very nice and generally better than CA parks.
Apps that help us: gas buddy, pilot and Loves. Allstays Camp & RV has a lot of information besides CGs and the app is better than the PC version IMHO. And there are others.