Leon,
Congratulations on a great and direct diagnosis. While I am sure you identified the problem, your fix may have a little tiny problem.
Was the tank coupling rubber? Most RVs of that period (and still) connected the vents with a tall rubber grommet. The reason the vent stack pulled out was just the coach flexing. It still is flexing, but you don't know what is happening down at the tank....
So, I will suggest that if it does have a tall rubber grommet at the tank, use the buzzy tool to clear away the foam and put a hose clamp on there. Then, go back on top and take your screw out. This will will allow the vent stack to float in the roof and it won't pull out of the tank.
I have come across a lot of this sort of thing since the depression killed my boat work business. It takes a lot of talking to get an owner to let you make holes in the coach walls.
Matt