Mine are actually in my laundry room (which would be a garage, if a previous homeowner hadn't finished the garage) and unfortunately is not air conditioned. It is definitely not as hot as outside, but it is warmer than the house. Probably gets up to the high 80's, maybe low 90s in there. I store RODI water, and salt water, in a 55 gallon food-grade plastic barrel, with a cover, and evaporation has been a non-issue.
I have two one gallon jugs of RODI that I keep in the freezer during the summer months, that I throw in the barrels before a water change that bring the temp down nicely. I then just re-freeze the 1 gallon jugs.
I would love to store it closer to the tank (I have a pump and 25 foot vinyl hose attached to my water mixing station, and simply pump it out to my ATO or tank as needed) but water barrels in the living room was not approved by the already quite understanding wife.