If you're getting black tank discharge after dumping and purging the line, you have a bad valve. They are not expensive and easy to replace. It might solve your problem. No one like a stinky water bay.
Tony & Ruth........... FMCA#F416727
2016 London Aire 4519, Freightliner chassis, Cummins ISX, 2018 Jeep Wrangler Unlimited, Blue Ox Avail with AF1. TST 507 TPMS