So, Bill-
Sorry to hear about your MISSED-adventure. I'm sure you have had many successful deployments in the past.
Now, we have only been doing this RV thing for almost 6 months, and when it comes to making thoes connections ourselves, we have made it our SOP to pull the Grey tank first for five to ten seconds to 'test' the connections are secure, and then pinch it off to save the rest of the grey for after the black dump. It seems to me, that it would be more desirable to get drenched in sudzy bath water on the first try than "The Nasty" coverings of last night's dinner passings...
...but maybe that's the real reason the grey sudzy water is saved for last... and not just to rinse the tubes!
I think thoes caravan predecessors sure are wiley pranksters by instructing us all to pull the black valve first. My condolences on the mess they had placed you in...