Hi, I had the same exact issue. Clearly a bad design.
I wish I had taken pictures when i was fixing it but I didn't so I will try to explain the process. It actually looked like someone had fixed it once before even though i purchased the coach new.
First I removed the old fitting from the end of the pipe which had broken off at the top of the tank. I bought another male threaded fitting and glued it on to the black pipe.
I cleaned the opening in the the top of the Grey tank then glued the pipes in place. Although I added a threaded male, I did not thread it into the top of the tank. Instead I used the threaded end as a taper that I could push and glue into the opening in the tank. I then applied liquid resin around the pipe and top of the tank to solid up the install.
I added metal strapping and bolted it down to the deck knowing that it would be under immense stress and would need as much support as possible.
Hope this helps!
Image showing the end I removed and replaced with a male threaded fitting:
Strapping around the fitting:
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