How's the air movement in the room? An oscillating fan to increase airflow between the room and the tubs would likely do them some good.
Also, where do you live that you have an ambient relative humidity of 65%? Above 60% starts feeling uncomfortable (IMO) and improves conditions for mold growth.
Is removing the "big tub of water" from the room an option? It sounds like lowering the humidity of the room would go a long ways towards lowering the humidity of the tubs.
This. Since it dropped 7% in that short amount of time, let us know what it falls to later on tonight. The decrease in the room should lead to a decrease in your tubs too. You're on the right track.
Last edited by Eric Alan; 06-13-2014 at 04:31 PM.
Reason: I spel goot