Five people were seriously burned by the explosion, which sent flames 200 feet into the air and the propane tank flying 150 feet away.
Among the seriously burned were the 42-year-old and her 18-year-old daughter who worked on the truck, which was parked near an auto body shop. They were both in critical condition.
Two other severely burned victims remained hospitalized.
Fire marshals, police and bomb-squad officers were investigating the cause of the accident.
Witnesses said the food truck -- La Parrillada Chapina -- regularly parks at the spot.