According to Evan last year there were 9 trailers, 4 from the Seattle/Olympia area, two from Portland, and one each from southern OR, CA and UT. As to facilities, he does not think there are any absolute requirements.
I have tracked the suggestions that have been submitted so far and done some research:
MaryHill St. Park, group camp seems to be booked, almost all other sites booked.
Grayland Beach Park, no group camp, other sites already booked full for the memorial day weekend.
Sequim State Park, group site is for tents only, upper loop has a number of spaces but they are only for tents, vans and small trailers. No hookups. We want to pursue this option; we will have to act quickly. $22 a night for either 2 or 3 nights. There are also hook up site left scattered between the two hookup loops.
Deception Pass, there are a lot of group sites but the all seem to be taken or are walk in sites. All other sites that can be reserved are reserved.
Big Cedars Campground, the county park day user from last year.
Hoh (Bogachiel?), do you really want to see how water proof you trailer is?
Kulsahn Campground. No reservations, no group camp.
Rocky Point Recreation Area, not sure what the arrangements are. Need more input from Bear.
From what Evan has said, having it in Western Washington, perhaps Central Washington, would work based on the demographics of the group.
It is increasingly looking like a state park is not going to be a viable unless we want to pounce on Sequim Bay.
I will keep looking.