I agree that volunteering does not sound like a 'gainful' employment. But Cali, NY and other such states are well known to take the most expansive view possible of your residence status and to pursue people who they believe are residents for taxes. They have been known to take legal measures and people have to hire lawyers to fight them off.
"A special division of the FTB has for years systematically targeted seasonal “part-time” residents for audit (I use the term “part-time” loosely, since we are talking about nonresidents who spend part of the year here, not part-time legal residents per se; but the term has stuck). Though Santa Barbara, Los Angeles and Sonoma counties experience their share of audits, historically the most common casualties are affluent “snowbirds” who own vacation homes in the Palm Springs area as an escape from the winter blasts of the Midwest or northern states. In fact, many of the major cases in residency taxation are eerily similar: they usually involve Midwesterners who own winter vacation homes in Palm Springs and environs. If the FTB finds significant taxable income coupled with meaningful contacts with California (such as a vacation home, business interests or long visits to the state), it can lead to the launch of a full-blown residency audit."
The part time residency trapRead that article Once again I urge you to call one or more of the places you are considering and talking to their bookkeeper for a local view of the situation. Are you really expecting to volunteer for more than 180 days?