Parshas Naso - Mishkan Duties of B'nei Gershon and B'nei Merari
I think that the answer can be found by examining the remaining topics of Parshas Naso.
The parshah introduces the requirement to send teme'im - those who are impure - from specific encampments. It then addresses the rules of me'ilah - unlawful use or taking of items owned by or dedicated to the Beis Ha-Mikdash. The parshah then turns to the rules of Sotah and Nazir, after which it presents the mitzvah and text of Birkas Kohanim and details the inauguration of the Mishkan, in which the Nesi'im brought korbonos and gifts.
Do these varying segments have a common theme?
The general message of the above topics centers around the need to stay on the correct spiritual course and not assume a position or carry on with a way of life which is deviant or out-of-bounds. Temei'm cannot be present in holy venues during their state of impurity, as entry to such places contradicts their status. They must know which places are appropriate for them when their bodies are impure. Me'ilah is a misappropriation of holy funds or property for personal use, whereby the line between personal and holy property is violated. The sotah has diverged from her marital status and bond of betrothal, crossing the boundary of permissible relationships. The nazir has deviated from normative behavior, and one opinion in the Gemara considers him a sinner for his actions. (The other opinion deems his lifestyle as noble; in any case, it is a change of course from a regular lifestyle.)
The families of Gershon and Merari were charged with transporting objects of lesser holiness. Their greatness of these Levi'im was that they did not protest that they should have been granted the right to handle the most holy vessels, which was the task of B'nei Kehas. B'nei Gershon and B'nei Merari were content to fill the posts assigned to them and not deviate from their missions, and this is their praise. Their fidelity to their charge in the face of any temptation to try to change course was the essence of their glory. This is the connection between the Mishkan duties of Gershon and Merari and the rest of the parshah, and it is precisely why the assignments of Gershon and Merari are reserved for Parshas Naso, rather than appearing in Parshas Bamidbar, along with Bnei Kehas.
The narrative of the offerings of the Nesi'im forms the conclusion of the parshah, for it details the appropriate manner of "deviation" in serving God. Although the Nesi'im brought their korbonos spontaneously and without prior precedent, they did not sacrifice them until Hashem authorized them to do so. (7:4-11) The Nesi'im wished to add to what is required in Torah observance, but they dared not do so without sanction. So, too, one who observes the disgrace of a sotah is encouraged to become a nazir as a deterrent to sin (Rashi from Tractate Sotah 2a). In this case, the undertaking to become a nazir serves as a reinforcement for Torah rather than as an alternative, new form of Torah life
It is no coincidence that Birkas Kohanim appears right before the offerings of the Nesi'im. The Ramban explains the proximity of Birkas Kohanim to the inauguration of the Mishkan, for Aharon himself recited it upon commencement of avodah therein. (See Ramban on 6:23.) The sublime message - as elaborated upon in great detail in earlier divrei Torah - is that the Mishkan exemplified adherence to God's will, so as to rectify the Chet Ha-Egel. Birkas Kohanim is thus the opening text of an eternal relationship of ultimate fidelity to God's commands, and it is therefore a fitting introduction to the offerings of the Nesi'im as the climax of Parshas Naso.