Black tank won't drain unless grey valve is open

DrewE wrote:
When both valves are open, does the black tank drain or only the gray water tank? If it's only gray water, everything points to a clog in the black tank drain system--a fairly straightforward problem conceptually, but definitely not a lot of fun to fix.

If both tanks empty, you have something plumbed very strangely indeed or have a very peculiar problem with the tank vent system.