My tub sits in an enclosed sunroom. The enclosure is mostly double glass sliding doors. I have a bathroom fan mounted in the ceiling that runs all the time. The only time I get any condensation is after a soak on a cold night.
I do have a 1500W heater in the room but it doesn't run overnight and if it's really cold (sub 32 degrees F), it doesn't have a prayer of keeping the entire room much above 40 degrees F.