The ornamental design for a propane tank sleeve, as shown and described.
FIG. 1 is a top plan view of a propane tank sleeve showing my new design;
FIG. 2 is a side view thereof; and,
FIG. 3 is a bottom view thereof.
The broken lines showing of the propane tank over which the sleeve of the subject invention is disposed and the artwork presented thereon are for illustrative purposes only and form no part of the claim.