Does the following idea have any validity? Is it brought down anywhere?
When members of the tribes of R'uven and Gad approached Moshe to unwaveringly request settlement on the east bank of the Jordan, apart from the rest of the nation, Moshe responded emphatically in the negative. He laid out conditions (which entailed participation in the upcoming conquest of the land west of the Jordan) under which they would be allowed to proceed with their bucolic dream. Once they had agreed to the terms, Moshe apportioned the land. All of this can be found in B'midbar 32:1-32.
But when it came time for the actual dividing up of territories, not only were the two tribes who had been involved in the negotiations allotted land, but another (partial) tribe was added - that of M'nashe. Since there was a dangerous endemic mentality among those two tribes who chose to split off for less than righteous reasons, Moshe paired them with a remedial force to counteract their divisive spirit. The tribe whose defining characteristic was unity and in clusion was was M'nashe, as exemplified by 5 of its members, the daughters of Tz'lafchad. They objected to the very hint of exclusion of their father's family from the rest of the nation (ibid. 27:4) because they possessed this crucial trait.