Since you're drinking water over a gallon a day and you're still thirsty, you need more salt. Everybody's needs are different, but to give you an idea, I also drink a gallon of water, and I need 1 teaspoon of salt to feel right. I mix it up in my water bottles and drink it all day long.
You might not need that much salt, but you probably need more than you're getting or you wouldn't still be thirsty. Salt and vit C will help stabilize your adrenals, which may be stressed given the changes you're introducing.