My apologies, I should have clarified that the second holding tank is actually the other black water tank attached to the second bathroom. It's marked "holding tank" on mine. It has a seperate release valve towards the rear of the rig and I emptied the black water but do not know where to flush that one from. There is a unmarked hose adapter on the opposite side of the rig, underneath the area for the refrigerator vent but when I hooked up the hose and turned on the water, water just squirts out of the sides of the white fitting.
Thanks on the tip about the strainers though. I'm going to get a couple of those!