What's a cold storage room? The amount of water that can be dissolved in
air is very temperature dependant. Let's assume that it's 75 degrees outside
and the relative humidity is 50%. That's nice, really comfy. OK, now lets
take this very same air and put it in the cold storage room. What happens?
Well the temp goes down and the humidity goes up. If the temp is 55 degrees
the relative humidity becomes 100%. Colder than that and you'll get water
dripping everywhere. So you need a door that seals well and a dehumidifier.
Your new vents are a bad idea. The air in that room must be much drier than
the air in the rest of your house (in Summer).
Try to rent a dehumidifier, install it, and seal the room. Check back in
a couple of days. If it's not much drier then you have leaks in your walls
or floor. Fix the leaks. If it's drier then buy a dehumidifier. I have no
advice on the musty smell other than a good cleaning with lysol or 5%