Thanks so much for all the great answers and, of course, a few silly ones.
I have been keeping the two grey water tank drains open, and want to continue doing so, which is the source of the back air flow when the wind blows. I do keep the black water tank valve shut and we celebrate Sundays by draining and flushing that tank. It's like a party!!! Yippee!!! But at least there is no poo pyramid as so eloquently stated above.
I had ordered all of the parts to do a makeshift connection to all three vents and vent the assembly through the building roof. I have to admit I was not excited about the prospect of punching a hole through my new steel roofing without knowing exactly where the plumber would do so later on in the project.
The big prize of the day goes to Lantley for the suggestion of the Air Admittance Valve. This is the best idea ever. I did not know such a thing existed. I just ordered three of them and I'll report back on how they work once they are installed.
Cheers everyone.