Is sleeping in university accommodation and eating for free considered forbidden (haram), knowing that I am a university student?

The reference for whether you are entitled to sleep and eat in university housing without paying fees is the university's regulations and conditions, in accordance with the hadith: "Muslims are bound by their conditions, except for a condition that makes lawful what is unlawful, or unlawful what is lawful." So, if the regulations do not permit it, then it is not permissible for you; but if they do permit it, there is no harm.