"Not content with doing thorough work in the land of Judah, the youthful ruler had extended his efforts to the portions of Palestine formerly occupied by the ten tribes of Israel, only a feeble remnant of which now remained. “So did he,” the record reads, “in the cities of Manasseh, and Ephraim, and Simeon, even unto Naphtali.” Not until he had traversed the length and breadth of this region of ruined homes, and “had broken down the altars and the groves, and had beaten the graven images into powder, and cut down all the idols throughout all the land of Israel,” did he return to Jerusalem. Verses 6, 7 . PK 397.1"
"And so did he in the cities of Manasseh, and Ephraim, and Simeon, even unto Naphtali, with their mattocks round about. And when he had broken down the altars and the groves, and had beaten the graven images into powder, and cut down all the idols throughout all the land of Israel, he returned to Jerusalem."
EGW directly quotes 2 Chronicles 34:6-7.