1 gallon a day from a 120 is not excessive evaporation, in fact it's kind of low. I add around a gallon of water a day to my 50 gallon corner tank, depending on the season.

Definitely use R.O. water. You can periodically check to see if you're losing so much salt from salt creep to noticably affect your salinity, but that's very unusual. If you're skimming like crazy and removing large amounts of skimmate, you'll lose some salt that way, but not much. If you're supplementing your tank with 2 part Calcium and Alkalinity supplements, you will slowly raise your salinity. That's because the Ca supplement is Calcium chloride, and the Alk supplement is Sodium carbonate. Your tank uses the Calcium and the carbonate, leaving the Sodium and chloride in your tank.