Lower temperature air, has less a capacity to hold water. As such, as the temperature drops, the humidity of the air comes down.
The lower the humidity, the more evaporation you will have from your tank. The more open water air, fans, lights, and water movement you have the more evaporation you will have.
I have a 150 gallon and lose almost 10 gallons a week.
Look into an auto top-off controller. Essentially you provide a container of fresh RO
water, and the machine automatically tops the tank up when necessary.
This will help to keep salinity concentration more stable.