When selling something nice, it usually pays to have it in top condition. Remember, you goal is not to recoup the cost of tires, a detail job, or any repairs or updating you do to get the rig in selling condition. Your goal is to sell the rig. Anyone buying your rig, or another 15 year or rig they are contemplating, is probably going to have to come out of pocket for the entire purchase price. Using the new tires as a sales pitch would probably be an advantage on a rig like yours. New tires mean less out of pocket expense for the buyer and gives them pause when comparing yours to a rig without new tires.
BTW, whatever you do, I wouldn't mention the current tires being nearly 8 years old with only 5000 miles on them. I would be concerned the rig had been parked for years on end and normal maintenance had been deferred. Out of sight, out of mind would be gnawing at the back of my brain.