That's not normal for a modern trailer. Pre-'74 or so, that was the way things worked. There was no greywater tank at all and the greywater was just piped into the sewer connection downstream from the black valve.
A 2000 Safari should have a greywater tank with its own valve. Maybe someone broke off the handle and just left the valve in the open position? Someone with a similar trailer should be able to tell you what it should look like, or maybe even post a picture of their handles.
My '75 Argosy has the grey and black handles in completely different places, one inside the bumper-trunk and one near the sewer outlet.
Zero Gravitas — 2017 Flying Cloud 26U | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill