"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".
I think hontou is much the same as "really (very)", or at least it's only differentiated between quality and quantity by speaking. Like, if you knock down someone's coffee, "hontounisumimasen deshita!" would be equivalent to "I'm really very sorry!" but "ho-ntou ni, sumimasen. warukatta na." would be more like "Truly, I'm sorry. I messed up."
Gauging by the number of Google results (not the best barometer, but without input from a native speaker, you have to go with what you have), すいた beats out すいている by a three to one margin. I also originally learned すいている, but evidently that is not the most common way it's said. Beyond that, I couldn't say more.
the verb is suku, お腹が空く, and so the ita and kimashita forms are both past tense, informal and formal respectively. Duo has been defaulting to increasingly formal constructions; but onaka ga suita is common enough that you may actually be learning grammar to avoid idioms here.
(BTW my dictionary probably auto-generates forms based on rules but I cannot wait to find this negative conditional form in the wild: "if someone weren't to become hungry"; "お腹が空かなかったら")