"I am really hungry."
Since no one answered, I will try my best to explain the confusion on the purpose of に here to the best of my knowledge and as simple as it can be.
| 本当/ほんとう/hontō = truth, fact, reality | It is categorized as a noun or an adjectival noun.
| 本当に/ほんとうに/hontōni = truly, really | It is categorized as an adverb.
The addition of に here changes the root form of 本当 to its adverb form 本当に, so it means "truly" here. に here in this case DOES NOT exist as a particle that denotes destination or target (although if you really wanna put it that way and ignore everything else, it literally means "to the truth"); ほんとうに is a united phrase that says "truly".