This is what has helped me:
Fill a spray bottle with a mix of 50% white vinegar and water. Spray the bags liberally inside and out, even where you donot see mold, because the spores and mycelia exist but are not visible to the naked eye. (You can also use a mixture of 30% rubbing alcohol and 70% water). Vinegar is acidic and should not harm the leather.
The entire bag should be sprayed well. Then, allow to dry outside, preferably in the sun. If you like, once dry, you can do this once more, if the mold is really severe. Then allow to dry well and in the open air, the smell of vinegar should disappear in a couple of days.
Once you are sure the bags are completely dry, apply a water resistant or waterproofing balm. Anything with beeswax should work. The beeswax will decrease the exposure to moisture which could again cause mold. The conditioning is important to preserve the leather. Ensure you rub in the conditioner on every part of each bag.Store in a dry area with a couple of bags of dessicant inside the bags. In the storag area, if possible, keep dessicants as well, or use rock salt.
Buy 2 plastic buckets which fit one inside the other. Punch a hole on the base of the inner bucket. Add some rock salt into this inner bucket. Rock salt is hygroscopic and the water collects at the bottom of the inner bucket. The hole in it allows the water to drain into the outside bucket. Empty the outer bucket periodically. This decreases the humidity in the area to some extent.
Hope this helps.
