my xm v6 rear rocker cover leaks oil. The gasket has had it. Do I need a new cover
and gasket?

Don't know anything about the V6 always ran diesels but in the normal scheme of things only the gasket needs replacing unless the cover has been damaged.

You can often make a replacement gasket by applying a bead of silicone sealant around the edge of the cover, after thoroughly cleaning it of course, then allowing it to set overnight before refitting it.

Peter