An often overlooked park in your area is Greenbelt (National) Park. If you like the DC area and want to spend a couple of months not too far away (and close to metro), they have camp host positions open this spring (March through May). The host spots have full hook ups. The hosts don't clean bathrooms, enforce laws, or collect money. You can find the listing at volunteer.gov.
An added benefit of volunteering in National Parks is the Volunteer Pass (America the Beautiful Pass volunteer equivalent), free to those who volunteer 250 hours in a year.