For years, Arthur "Mr. Okra" Robertson drove his folk-artsy painted truck through downtown neighborhoods selling fruit and vegetables. He is the focus of a recently debuted documentary film. But he may have parked his truck at Jazz Fest for the last time: The engine died. A benefit at d.b.a. (618 Frenchmen St., 942-3731; www.drinkgoodstuff.com) Thursday night will help him buy a new truck. Performers include Morning 40 Federation, Happy Talk Band, Guitar Lightnin' Lee, King James and My Graveyard Jaw.