。
She thanked him from the bottom of her heart.
Literal
She [topic-は] him [to-に] sincerity infusing thanked.
真心 ('sincerity, true feelings, heartfelt-ness') is one of those Japanese values-laden words — implies pure, unselfish, deeply felt emotion. 真心を込めて ('with sincerity infused') is a set adverbial phrase where the を is sometimes elided in writing (真心こめて). The te-form 込めて modifies the verb (感謝する) as manner adverbial.