Rashi on Numbers
ונסע אהל מועד AND THE TENT OF MEETING SHALL SET FORWARD after these two divisions (those of Judah and Reuben), as is shown by the position of this statement in the narrative.
Sforno on Numbers
ונסע...אהל מועד...בתוך המחנה. Between the two flags which have already been mentioned.
Rashbam on Numbers
בתוך המחנות, two flags (six tribes) would journey at the head followed by the parts of the Tabernacle and the Levites. These were followed by the last two flags (six tribes, there being a total of 4 flags.)