National Park volunteers...

Last summer I took the scenic route to Denver and stopped at the Gila Cliff dwellings near Silver City, NM. Working the site were three older people. I spoke with one of them, and he said he was a volunteer along with his wife. They apparently worked there for free for several months in exchange for free room and board (I'm unsure if they were there in an RV or used park facilities). The guy said they'd done this at a couple of different parks.

A quick Googling showed me a volunteer.gov site, but that seemed rather sporadic. Is this the right place to be looking, or is there some other specific program that sounds like this?