Sorry to hear of your problem and seeing as I use a Sewer Solution 99% of the time, I no longer have recent experience with the hoses you're using.
That said, when I DO dump the tanks with the stinky slinky, I always pull the gray tank first, make sure there are no leaks and upon verifying that, I close the gray tank and open the black tank valve. Then when the black tank is emptied, I close the black tank valve and I drain the rest of the gray tank.
Yes, I've had times when everything looked good yet the hose still leaked and I found it's much better to work on if there's gray water involved instead of black water.
Good Luck,
~Rick