I have no idea about the shower backuup, but when my tank is full the first indication is the shower. Obviously, you have proven that you still have 1/2 tank of capacity left. Could the shower have it's own connection into the tank and that connection extends downward to the 1/2 tank level?
Regarding
The other thing I've noticed is that when I dump the grey tank the first 1/2 to 3/4 drains quickly and then seems to slow to nothing, then, suddenly the remaining 1/4 tank will let go and drain. Any ideas?
Mine does this, and always has. There was another thread about this recently, and many responses said their tanks drained the same way, so this is not uncommon.
Let us know what the final solution is determined to be.
Fred