Where is the gray water holding tank drain valve?

[ucolsbl]

I recently purchased a 2008 Cardinal fifth wheel by Forest River. I have lived in for about 2 months and I am loving it.
Recently the water stopped draining from my kitchen sink. The toilet and bathroom sink drain fine. Is there a separate release lever for the kitchen sink's gray water? If so where is it?
Thanks for your help.
Blanche
in Oregon