Is the entire length of a B747 or A380 wing used as a fuel tank?
Fuel tanks take most of the space in aircraft wings, but not all. There are other things on wings too. They are ailerons, flaps, slats, air brakes. On most airliners, the engines are attached to the wings too, so their assembly takes up space.

On the two aircraft of your interest, here are the pictures:

A380 (its nickname is not jumbo, but superjumbo)

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747

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