If we're on the same page, the low point drain near the rear of your rig is the drain to the fresh water tank. Without the cap on any water you send back there will just run out on the ground. With the tank empty there isn't any way to create suction and so your pump won't be able to do anything.
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Dick, Joyce, Diego, Picatso and Gustav
2017 3720 RL, and 2013 HC 343RL
Pullrite Hitch, IS, Disk Brakes, 3rd AC, Winegard Traveler, Bathroom door mod, Dometic 320, couch for desk swap, replaced chairs, sun screens, added awnings, etc.
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