Black water flush issue

[Ggeoffrey]

I have purchased a used 2012 Autumn Ridge last fall I had someone winterized it's for me. When I opened and used it first time I hooked up black water flush, opened the black water valve outside, and the fresh water flooded under my bathroom sink. There were no issues last year with the back wash. Is there a valve inside the trailer that needs be opened? The water was coming out of what looks like an overflow bypass but I could find nothing in the manual.
Help if you can. Until I fix this I will back was black water via flushing the toilet.