Not a van-specific event, but when asking the host of the 2017 event about reserving the field for one, he gestured to all the space available on the sheep pasture and asked, "why not invite them to this?" It will be the 16th Centralia Campout, and it's worth knowing that it was originally conceived as a farther-west and completely unstructured version of the Weiser festival in Idaho, so there are lots of old-time music players. If you aren't a musician who wants to participate when the urge strikes, if you come you'll want to appreciate and respect this vibe to have a good time.
It's a wonderful time, right on the river (swimming and floating). In brief: potties+a couple toilets+a couple showers, several wash stations, covered potluck area, a covered dance pavilion (where they call a couple of squares and contras), leashed well-behaved dogs allowed, no fires, and great unplanned, intermittent old-time jam sessions around the field. It's a 20-min walk to town or a short bike ride. Oh, and a taco truck on site. So let's go! http://centraliacampout.com/
So there are no surprises, and in case you skipped past the link, this is spendier than your average camping experience. Last year was $80 per person for the week, or $20 pp per day; children 11 and under are free (cash prices; others pay 10% more). Be prepared.
For even more info, photos, videos, ask to joined the closed group on Facebook: https://www.facebook.com/groups/centraliacampout/