You are supposed to register vehicles/campers, but registration is closed so you’d have to buy a vehicle pass from someone that already has it.
Otherwise, I don’t know how closely these are checked. Worse case, you wouldn’t get to go into the “official” campground and would have to find a place on your own. I wouldn’t think that would be too hard, but never tried it.

Hum, my post didn’t post? I’ll try again
You are supposed to have a vehicle pass for your vehicle/camper, but the registration date has passed. If you want one, you’ll need to purchase one from someone that already has purchased it. Should be doable.
However, I don’t think vehicle passes are checked too often (or so I’m told). Best case, no one checks and you can park wherever you want. Worse case, you can’t go into the designated parking area and will have to find your own spot, which I wouldn’t think would be too difficult.

The posting “flash to bang time” here is sometimes quite lengthy.

RIDE RIGHT