The ornamental design for a boattrailerfendercover, as shown and described.
FIG. 1 is a perspective view of a boat trailer fender cover, showing the new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right-side elevational view thereof;
FIG. 5 is a left-side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines showing a boat trailer in FIG. 1 illustrate environmental matter while the invention is in use, and form no part of the claimed design.