This isn't a park its considered a national forest your best bet is going to be contacting the forestry department for riderside county and ask. it use to be open to on hwy vehicles but they ended up blocking the path with a giant locking door and the only way to gain access is through the rangers.
Not unless you want to get fined. Also, you may want to plan ahead, as some campsites are booked well in advance.
I think you will have to pay.. we had our dog with us. I think they have a limit on days. We have found it difficult to get a hold of the Rangers in the California park system
Most national parks have a fee I would call and ask
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