new to campers. kitchen sink wont drain, grey tank is empty

Just bought a 2008 Fleetwood terry 295tsrl a week ago and been living in it. This morning my wife started doing dishes and went to drain and it went down about 3" and stopped. Don't wanna unscew the vacuum breaker for fear of it filling my cabinet with water. Went ahead and drained the black and grey tanks and the sink is still full. From reading online it says this thing has 2 grey tanks. But the underside is covered with just the 1 grey and 1 black line coming out and connecting. Then at the back there is a small 1" or so drain I'm assuming is for fresh water as it is directly below the fresh water tank. Went ahead and drained it as well still nothing. Other than that there are 2 small drains with screw on caps and that's all that's below the cover.

Is any body familiar with these or where a second drain valve is located?

Any help is appreciated. Thanks


Also it is a 5th wheel if that makes a difference.
The level indicator says empty on grey and black and full on battery and galley