The ornamental design for a fresh watertankunit for a portabletoilet, as shown and described.
FIG. 1 is a left, front perspective view of a fresh water tank unit for a
portable toilet showing my new design, on a reduced scale, the broken line
drawing of a toilet and housing is for illustrative purposes only and
forms no part of the claimed design;
FIG. 2 is a left front perspective view thereof;
FIG. 3 is a left rear perspective view thereof;
FIG. 4 is a right rear perspective view thereof; and,