Because urine is so corrosive, any metal black water
tank that's more than a couple of years old is living on borrowed time...so leaving the tank full for the winter, even without salt, is not a good plan. Adding salt certainly won't extend the tank life...and depending how cold it can get and how much is in the tank, it may take a LOT more than just a couple of pounds to provide any protection from freezing.
So you really do need empty the tank AND thoroughly flush it out before the boat sits...no easy task, judging from the size of your boat. But if you can get a portable pump and water hose to it, or a barrel to catch discharge from a macerator pump, I urge you to do it.