I claim the ornamental design for a multi-tank utility trailer, as shown and described.
FIG. 1 is a perspective view of the multi-tank utility trailer;
FIG. 2 is a right side view of the multi-tank utility trailer of FIG. 1;
FIG. 3 is a left side view of the multi-tank utility trailer of FIG. 1;
FIG. 4 is a front side view of the multi-tank utility trailer of FIG. 1;
FIG. 5 is a back view of the multi-tank utility trailer of FIG. 1; and,
FIG. 6 is a top view of the multi-tank utility trailer of FIG. 1.
Any subject matter shown in dashed lines, and any surfaces of the design not shown, including interior surfaces, form no part of the claimed invention.