They are made of synthetic material, I don't think there is a problem with them drying naturally. The only issue I see is perhaps mold growth when rolled up wet.
When we use our coach, the day before we move to a new location, we check the weather reports. If the forecast is rain, we retract the slide out that evening while it is dry.
Fortunately, we can use our rig with the slide out in or out - this is not possible with some RVs.