The ornamental design for a motorcycle fuel tank, as shown and described.
FIG. 1 is a side elevational view of a motorcycle fuel tank showing my new design, the opposite side elevational view being a mirror image;
FIG. 2 is a top plan view thereof;
FIG. 3 is a frontal perspective view thereof; and,
FIG. 4 is a rear perspective view thereof.
The broken line showings of the element on the top and its boundaries, and the oblong portions along the front, are for illustrative purposes only and form no part of the claimed design.