The Rambam in Sefer haMitzvos shoresh 3 writes that the din of “lo ya’avod od,” that a Levi who is past 50 years old can no longer do the job of carrying, is not counted as a mitzvah l’doros. Ramban takes issue with this idea, as we see that even later in Tanach whenever the aron was moved it was carried on the shoulders of Levi’im. The Brisker Rav (michtavim at the end of Ch haGRI”Z) writes that there are two dinim in carrying the aron: 1) there is the mitzvah of carrying the klei hamikdash, including the aron, which the Levi’im are charged with in our parsha; 2) there is a separate mitzvah of carrying the aron – “Ba’eis ha’hi hivdil Hashem es sheivet haLevi la’seis es aron bris Hashem…” (Devarim 10:8) The Rambam rejects “lo ya’avod od” as a mitzvah l’doros as there is no din l’doros of moving klei hamikdash, especially once the Beis haMikdash was built; however, the Rambam in his count of mitzvos aseh lists carrying the aron by kohanim (not Levi'im) as a seperate mitzvah. Ramban does not disagree as a matter of principle that carrying the aron is a seperate mitzvah. However, he argues that when the Levi’im perform this task, all the requirements of avodah by levi’im mentioned in our parsha are in force.
It could be that the Ralbag deliberately does not mention the klei hamikdash as a takeaway l’doros, but mentions carrying the aron in keeping with this idea that only carrying the aron is a mitzvah l'doros. The only little problem I have is that the takeaway l’doros belongs in parsha Eikev, not here.