The Germanic tribes were not nomads, they were farmers. As they moved into the Empire, they
brought with them their own farming techniques, including a new type of plow well-suited for
farming the heavy northern soils. They brought also their own style of land-holding, settlement
patterns, and diet. Once they had fought their battles and won their land, they pretty much
settled down to being farmers and lords of great estates.
They were new lords, of course, so there was a certain amount of displacement of the existing
landowners. As I've already noted, at the lowest levels what we see is a slow but steady change
from a countryside populated by slaves to one populated by peasants and serfs. The landowners
themselves gradually inter-married, so that there was less and less distinction between the Romans
and the Germans.
The barbarians affected the cities of the Empire only slightly. They might sack a town, and the
immediate effect was traumatic, but the town existed for certain reasons, and the coming of the
Germans did not change those reasons. Some towns were administrative centers, others trade or
manufacturing. The need for trade and manufacture might decline, but it still existed. The
administrative centers collapsed more quickly, but this had more to do with Imperial politics than
with the movement of tribes. Most towns recovered quickly after being looted; indeed, certain town
were looted many times over, attesting to their continued ability to generate wealth.