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

I used to have a Keystone Outback with two grey tanks and the second handle was under the slide (driver's side) between the wheels. PITA to get to when set up in a campground. Your monitor saying that Galley is full would definitely indicate that you have another tank to find and drain.

The two small drains with screw on caps are the "low point" drains for hot and cold systems. Used when winterizing the trailer.