Good to go
Mayfield Lake Campground is owned/operated by Tacoma Power utilities (TPU), thru the Washington State Parks system. TPU is an entity of the City of Tacoma (municipality) and as such RCW 9.41.290 (Premption) applies.
I had an issue at another TPU campground last summer (Alder Lake), and TPU resolved the issue, removed the incorrect signage, and there was no issues this year when I camped at Alder Lake over Memorial Day Weekend. You should have no problems at Mayfield Lake as far as OC is concerned.