White Substance on Small Screen Next to Water Fill

Those fresh water tank vents should be outlawed. That scum, bird droppings, wash water, rain and whatever else, goes right in your drinking water. I put my intake and vents behind a locking door.

When you sanitize a stupid system like this, use plenty of bleach and fill the tank so it overflows both the fill hose and the vent. Let it set for at least half a day.