How to find campgrounds

Just got my trailmanor. It is used and I want to go to a campground with hookups to test it out.

While I expect to do a fair amount of boondocking, I wanted to know how to find campgrounds, and if there is an organization that tends to have more reliable services?

For next week, I wanted to be in the Riverside county area of California. Preferably in Corona area of Riverside county.

Thanks in advance.

Michael