'Volunteering at state and federal park

We have been full timers for the past couple of years. My wife finds everything on volunteer.gov. In all the cases you do need a background check. Our first assignment was with the Corps or Engineers who require a full Background check. Sometimes this can be transferred to other state or federal parks or preserves. A lot depends on the park or organization. You also have to fill out a application for all you are interested in. Later on, as you gain experience you can attach a "volunteer" resume. As noted from previous entries, there are Contractors and Volunteers. There is a difference. We have chosen just to volunteer. Most parks require around 24 hours service per week in exchange for the camp site.
Good luck and enjoy your experience.