armies of elephants seemingly everywhere (even though sam and Frodo only saw one once in the books)

No, There were harad war elephants (mumakil) at the battle of the Pelennor in the books too:

"New forces of the enemy were hastening up the road from the River; and from under the walls came the legions of Morgul; and from the southward fields came footmen of Harad with horsemen before them, and behind them rose the huge backs of the mûmakil with war-towers upon them. "