The Hunas are mentioned at least 6 times in Mahabharata.
In Mahbaharata 2.29.11(critical edition), when Nakula meets Hunas after crossing the Sindhu river in the north. He subdues them
रमठान्हारहूणांश्च प्रतीच्याश्चैव ये नृपाः |
तान्सर्वान्स वशे चक्रे शासनादेव पाण्डवः ||११||
The Hunas are mentioned once again, along with Chinas and Śakas in Mahabharata 2.47.19.
They pay tribute to Yudhishthira and attend his Rajasuya.
वार्ष्णेयान्हारहूणांश्च कृष्णान्हैमवतांस्तथा ||१९||
They are again mentioned in Vana Parva.
Hunas and Tusharas are both mentioned in Mahabharata 3.48.21 along with Chinas
हारहूणांश्च चीनांश्च तुखारान्सैन्धवांस्तथा |
जागुडान्रमठान्मुण्डान्स्त्रीराज्यानथ तङ्गणान् ||२१||
They are mentioned again in Bhishma Parva (6.10.64).
It is very hard to dismiss all these references as interpolations, which some people do. These references are scattered across chapters. They are present in all versions of the epic across all recensions of Vyasa Mahabharata. Interpolation is not tenable by textual evidence.
We have to assume that these references are central to the epic. Does it mean that Mahabharata in its final form complied in the 5th century? Certainly not. We see in all these verses that Hunas are placed along with Chinas in Central Asia. They had not invaded India by then.
According to researcher Christopher Atwood, the Xiongnu people of Central Asia were known as Huns at least since the 2nd century BC. It could go further back in time though we currently have no evidence for it.
The conquest of Hunas & Tusharas by Pandavas by Mahabharata does not mean that Vyasa actually describes conquests of Gupta kings of the 5th century. Likewise, the conquest of Hunas in Raghuvamsha does not mean that Kalidasa actually describes conquests of Gupta kings of the 5th century.