What are th chances of getting a campsite that’s “walk in only” if I show up on a Friday morning?

Good

First come , first serve sites that are available go on sale at 8am each morning.

If you are able to call them might have a better chance. Sure you would find something. Early

Iffy

I've always had great success

Iffy simply because schools are out and its a Friday. If you can, try for Thursday instead, you'll probably have a better chance at a better site then on a Friday.