Welcome to the forum
When you bought your trailer, did you get any kind of walkthrough showing you what everything was?
You can hook a hose to that connection, turn in the water and then look at the monitor panel, watching for an indication that the level is rising in the rear black tank. Just don’t let it run any longer than needed.
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2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.