I claim the ornamental design for a trailer tongue cover, as shown and described.
FIG. 1 is a left side view of the trailer tongue cover.
FIG. 2 is a front view of the claimed design.
FIG. 3 is a rear view of the claimed design.
FIG. 4 is a top view of the claimed design.
FIG. 5 is a bottom view of the claimed design; and,
FIG. 6 is a perspective view of the claimed design. The trailer and jack of FIG. 6 are depicted in broken lines, which is environmental matter that forms no part of the claimed design.