Sewer vent pipe off

We have had a problem with sewer gasses coming into our Rushmore coach when dumping ever since we got it in 2011. Finally took the pocket door off and yes, the exhaust pipe has come apart where the bottom connects to the part going through the roof. How do you re-align these two pieces and re glue? The disconnected part is near ceiling.

If you had a four foot skinny arm it would be a piece of cake.
But since most of us don't, being creative comes into play here.
I would go up on the roof, remove the mushroom vent cap and try to pull the vent pipe up a bit.
If that works, then fabricate a method to attach a PVC glue dauber to some sorry of stick.
Then fabricate some push-pull sticks that will reach the pipe.
Now the fun part.
Take your dauber full of PVC cement, apply the cement liberally to the pipe fitting.
Having one person on the roof pushing down on the vent pipe, guide it using the previously fabricated push-pull stick.
Once in place, reinstall the vent cap.

Peace
Dave