The Bible shows that Aaron and his sons were chosen from the rest of the sons of Levi to officiate as priests, and these "other" Levites had secondary functions in religious service. There is, however, something I can't see clearly yet: Priests (Aaron's sons) would indeed enter the Court and the Holy Place, but would other Levites also enter the Holy Place?
(Of course, not in the Holy of Holies, for that would be only once a year, only the High Priest, and therefore a son of Aaron.)
Numbers 18:4 made it more complex, for it says that Levites were to "keep the charge of the Tent of Meeting, whatsoever the service of the Tent may be," and the following passages don't help either. Especially verse 7, where it states "thou [Aaron] and thy sons with thee shall keep your priesthood in everything that pertaineth to the altar, and to that within the veil," this veil is the veil of the Holy of Holies, suggesting that the "other" Levites could also work inside the Holy Place, and not only the outer court.
Other passages, such as Numbers chapter 4, suggest that Levites wouldn't enter the Holy Place etc.
So: Levites could enter the Holy Place or not?