I know this isn't addressing your immediate problem but have you ever considered using the equalizing-method to transfer volume from the gray to the black tank in order to free up more space in the gray tank?
We used it frequently and it kind of eliminates the need to wash your dishes in a tub and then go and empty it in the toilet which may possibly have prevented what happened to you.
There are lots of threads here on the forum on how to do it and several youtube vids on it if you're interested in giving it a try.
You are transferring the gray water to the black tank so there's no movement of black water involved ...just gray water equalizing because you will always make sure that the gray tank has more volume when doing the procedure. We even did it for awhile without adding an extra knife valve and just collected the trapped gray water in a coffee can.
Here's one video: