FIG. 1 is a perspective view of a robotic lawnmower deck, showing the new design; FIG. 2 is a top view thereof; FIG. 3 is a right side view thereof; FIG. 4 is a front view thereof; FIG. 5 is a back view thereof; FIG. 6 is a left side view thereof; and, FIG. 7 is a bottom view thereof. The broken lines in FIG. 7 are included for the purpose of illustrating portions of the robotic lawnmower deck and form no part of the claimed design.