The Pacific Coast Highway is US 101 in California. You didn't say how you were coming into the Bay Area, but I suggest you don't take 101 through San Francisco. Highway 280 from the south (San Jose) would be preferred. If you come in from the east, use 580 to get into Marin County.
There are 2 RV parks in Marin County, just north of Mill Valley, which is where you pick up the access road (Shoreline Rd, also Hiwy 1) to Muir Woods. They are:
1. Golden Gate Trailer Park, Greenbrae, CA, Last year's rates: $33, Visa, MasterCard;
2. Marin Park, Larkspur, CA, Last year's rates: $38, Visa, MasterCard; firstname.lastname@example.org
Both parks are right near Highway 101 and may you may be exposed to highway noise.
There are other parks further north of these 2 parks, but the distance and possible commute time issues may not make them convenient for you.
There is now a shuttle bus available to take you to Muir Woods. They are trying to limit private vehicle traffic because it gets really congested in the summer months, the road into the park is narrow, and the parking is limited. You may want to check into that. Here is a summary: "Golden Gate Transit will operate shuttle service between Marin City and Manzanita Park and Ride to Muir Woods on weekends / holidays between Memorial Day and Labor Day. Fares will be $2 for adults, $1 for seniors, disabled and youth (age 6-18). Passengers under 6 will ride free."
Enjoy your trip. If you need additional information please ask. We lived in the area for 26 years before retiring to a less congested area of California.