Steve, two things cross my mind.
1. Did the shower ever drain ... or have you always had to have both the shower drain valve and the gray water drain valve open?
2. If the shower used to drain, but doesn't now, it's possible some mud daubber wasps built a nest in your grey water vent stand pipe. Water can't enter unless air has a way of escaping. So pop the cap off the top of the gray water tank stand pipe (on top of the Casita
and have a look down it with a bright light
3. If it's never drained, it's possible the rocket scientists at the Casita
factory shoved the grey water stand pipe too deeply into the grey water tank ... causing even a minimal amount of water in the tank to cut off the air escape
up the vent stand pipe.
I've heard of folks drilling a small air escape
hole in the tank near the PVC stand pipe, just above where it enters the grey water tank. A little hole will let a lot of air escape
... and won't cause your gray tank to leak until it's full to the tippy top.