Agreed. Downieville is a good one.
The other possibility is right where Highway 49 meets Gold Lake Highway. This is about 3 miles from the entrance to the campground, and there is a general store with restrooms, a small diner, and fuel / water.
Although, it's not the cheapest place to buy fuel, and there's limited cell phone coverage. Downieville definitely has more restaurants, etc.
Speaking of which, how many are planning on coming up Friday? I'm figuring I want to be to the camp entrance no later than 2 or 3 to leave plenty of time for setting up camp. That would put us in Downieville around 1:00.