No advance reservations are needed and accommodations are on a first come first served basis. If you pull up and see an open spot, you can take it wether you are in a tent, trailer, Class A or Skoolie. Meriwether Lewis Campground allows for a maximum of 14 consecutive days, and a total of 30 days of camping per year. There are toilets with running water, 2 water spitgots throughout the campground, and the sites are all paved. There is no water, sewer, or electricity at the sites.
More information here: http://www.exploringthelocallife.com/boondocking-review-5-things-meriwether-lewis-national-monument-campground/