Ever think of him getting a new one?
Of course, BUT, in ROTJ he is clearly flying Red 5, with the 5 red stripes on its wing. If he did get a new one - he must have been nostaligic enough to paint it the color of a squadron that no longer exists. Red squadron from ANH was later renamed Rogue Squadron, with Luke as its leader. By the time of ROTJ/Luke becoming a jedi, Rogue Squadron was led by Wedge Antilles of course. So, in the Battle of Endor, there would be a Rogue 5(im not sure if this ship/pilot made it on camera) which would have been a ship exactly the same as Luke's(decked with the 5 stripes). Luke had left his own XW aboard the Mon Cal ship, Home One, after returning form Dagobah.
Much later in the EU(around Black Fleet Crisis Time) I think, there is a comment where Luke is reflecting on how he has had his XW for 20 years or so.... So yeah, that gap is still there