You've got a leaking grey or black tank valve. With the SaniCon connected to the 3" discharge, you won't notice a leak, but if you had it capped off, every time you remove the drain cap, you would have build up, behind the cap. The leak is going through the bypass and filling the hose.
Easy to check, remove the SaniCon 3" hose, close the valves and leave a drip pan under the 3" discharge.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor