The story is BS...
Motorhomes have a dump tube with a cap and a slide valve. There is a slide gate valve for both grey and black water. They share the same outlet tube. There is no top access to any part of the sewage system unless you try to siphon right out of the toilet.
Those stories circulate because 99% of the people who read them don't really know the real details of the subject.
It would still be funny if they changed the story to where the guy uncapped the dump, pulled the gate valve and covered himself with sewage...
Why be a killjoy? Does it bother you that people might get a laugh outta somethin?
I actually think it funnier that they got the facts so blatantly wrong and then made it look like a newspaper article.
Debunking urban legends is usually funnier than the story of origin anyway.
The one about the lady, her dog, peanut butter and her friends hiding in wait to surprise her for her birthday...True I hope!