We use a "Y" and have for a few years now. In fact, I was all proud of the "Y" on my "Y" so that I could feed my 2nd black tank flush. Problem was- it was a cheap Harbor Freight (convenience purchase vs. looking for a cheap price)... it leaked and I almost found out too late when my wife flushed the toilet and water gushed UP into the toilet.
Now I don't leave my black tank flush hoses connected. I'll still use the "Y" so that I don't have to disconnect my fresh water hose, but I'll quick connect when the black tank flush is needed and disconnect when it is done.
