Dear colour, this quenstion has been asked and answered many times. Do a search.
Do not set a %RH too low as the condition would dry out the seals, lubrication of your lens(s), camera(s) & other equipment and render rubber brittle and would crack. In certain dry cabi manuals, the recommended range would be around the range of 40-60%.
Fungus spores germinate in approx 55-60% %RH, but I keep mine around 45%. As I do open my dry cabi often, the %RH would be in the average of %50. Unless I put in my gear after use, the %RH would go up to around 60%, but because of the difference with my settings and the %RH in my dry cabi, the unit would bring the %RH down faster to around 50% (Way within the spore germination range of 55-60%), then slowly drop to 45%. Have used this setting for years and have not encounted any problems with my gear.
I have read that UV does "kill" fungus spores, but IMHO, I do not think so. Spores can drift and spread in our atmosphere (floating in the atmosphere, being exposed to the sun and all), thus proving that UV cannot harm them as when the conditions are ideal for them to germinate, they will & do.
Any dry cabi would not totally prevent fungus, it just prevents the spores from germinating by ensuring an environment that isn't ideal for the spores to germinate. There are already fungus spores in your dry cabi now, like in everyone's elses. Once we use our equipment, they are exposed to spores (i.e. from the atmosphere, our palms etc) people have kept their equipment in dry cabis for years and are alright is because the conditions are non-ideal for the spores to germinate.
Don't worry too much about fungus or spores. Just use your equipment often. That is my best advice & the best way to prevent fungus.