I remember reading that Moses gave one tribe of Israel the task of erasing every trace of the Israelites' journey from Egypt to the Promised Land. Is this accurate, what is the source, and what would be accomplished by such an exercise?
Possibly based off of the duty of the Tribe of Dan, as stated in Bamidbar 10:25:
"Then, as the rear guard of all the divisions, the standard of the division of Dan would set out, troop by troop. In command of its troop was Ahiezer son of Ammishaddai."
And explained by Rashi:
The Talmud Yerushalmi Eruvin 5:1 has the following statement: Because the tribe of Dan consisted of numerous troops (i. e. was more numerous than each of the other tribes, except Judah which marched on the front, cf. Chapter II), it marched in the rear so that if anyone of the other tribes lost anything, he (Dan) restored it to him. (מאסף לכל המחנת therefore means “the one who gathered in everything belonging to all the camps”.) etc.
Further explained by the Natziv:
מי שנחשל ולא יכול ללכת עם מחנהו. והי׳ נשאר יחידי אחורי המחנה. נאסף למחנה בני דן. ומחנה בני דן היו הולכים לאט עם הנחשלים עד שהביאום לצבאותם:"
Translation: Whoever that was weak and couldn't walk along his camp, and remained by himself behind his camp, would be assembled into the camp of the Sons of Dan, and the camp of the Sons of Dan would walk slowly with the weak until they brought them to their troops.