I have worked as a volunteer at Yosemite National Park as a campground host in the Winter.
It was only a few hours a day checking people into the campgrounds, walking a bit through the campground, and occasionally dealing with bear problems. My wife and I currently and for the past nearly 20 years work for pay in Yosemite as well. Yosemite has many volunteers doing many things and provide many of them with a campsite and in my case provided propane and a full hook-up site. They have a department that deals with all the volunteer positions. That would be the case in most national parks I would imagine, and they would be the ones to talk with. Good Luck!