I've been hosted from couchsurfing.org in SLO.
Several years ago, when I rode up to the Oceano campground and learned it no longer had a hiker/biker site, I ended up in a small county campground that was just around the corner. I paid full price, which was the same as it was at Oceano. If my dim recollection is still accurate, spot could be reserved at the county campground, as well.
Because of the Dunes, where people can ride dune buggies on the beach, there are lots of RVs and big trucks in Oceano. This is the main reason camping is at such a premium.
Another option, besides the hostel in SLO and couchsurfing.org, would be to use the hiker/biker site in Morro Bay, though that would make the next day's ride to Lompoc (or wherever) a long one.
Ray