[wingnut60]

The valve cable may be separated--for some reason the valve is not opening no matter what the 'feel' is--can you trace the drain lines back to the valves? OR, there is a blockage in one of the lines itself.
First, do both of the grey tanks empty out of the same drain? If so, the blockage must be downstream of the junction of the lines, or it would back up into one area only.
Don't see any way but to expose the grey drain line(s) in the underbelly and start disassembling. Not fun, 'cause everything is put together glued....if you have to cut a line, keep in mind that you can get rubber connectors at Lowes/HD that clamp lines together instead of trying to re-glue.