those are some pretty scary numbers. I remember reading that in actual fact, if the numbers spoken of were to have realistically travelled according to what was possible back then over known terrain in known groups, then for the forty odd years they supposedly camped out in the desert, not once would the entire procession be able to stop in one place - as you say, by the time night falls and the guys at the front stop and sleep for eight hours, the guys at the back are only just arriving at a campsite...which is now vacated by the guys at the front.
So out in this desert, where they were being relentlessly persued by an army at a snails pace which allowed them apparently decades-worth of potty breaks, if they actually all stopped at once it would be a semi-permanent dwelling-mass as large as a whole country/state of the USA which would have left an indelible mark on the countryside. In short, we'd be picking up middens, cartwheels, firepits, pottery, bones, blankets, skeletons, lost socks, false eyes, wooden legs and all the other detritus that humanity drags around with itself. These sites would be there for centuries, millenia even.
So...where are they?
quite apart from the fleeing jews, you've got three things happening:
1) egypt has been decimated by about a decade of famine and pestilence, culminating in the death of some large part of the population (anything up to let's say 1/4 of the population under 18)
2) egypt just lost faraoh and his armies
3) and on top of that, egypt just lost it's captive workforce, the guys responsible for planting crops, repairing buildings, emptying toilets, manual labour and all the dirty jobs that a civilization like egypt requires to function on a day-to-day basis
so, you've decimated the (non-jew) population. you've destroyed her armies and her ruler and gelded the workforce - AFTER a decade of let's say "biological warfare".
so, what do you think happens to a country with no workforce, no soldiers, no leaders and no telephone sanitizers?
that's right - total collapse. civil war, bloodshed, extinction.
So, er, where's this period of failure where the egypt we know and love ceased to be for, oh, let's say 300 years at least, if not for good?