The vent problem sounds like you may be right. It could be that you are actually pulling the water from the shower drain trap, and once you put enough draw on it while draining the gray tank, you hear that woosh sound....of air being sucked in one of the drain traps.
As far as I know (you will have to find your own gray tank vent), the gray tank vent is on the tank itself, not running up to the roof of the RV the way the black tank does. If you can get under the RV and inspect the gray tank, you may find the vent, and see if there is anything that is obstructing it.
Black tank odor may also be coming from the shower drain, if my first thought is correct, and some of the sewage vapor has gotten up into the gray tank line, and then up through a drain without water in the trap.
It could also be that your roof vent IS plugged, but even then, you shouldn't smell the black tank, unless there is a crack in the vent line, or your toilet seal is failing. Will water stay in the toilet, or does it drain down into the tank?
Also, sometimes, if driving down the road, wind will push air down into the black tank, and in effect, "pressurize" the black tank, causing stinky air to be forced up through the toilet seal.
If this is a problem, you can buy items that go on the top of the vent, to help cause the air to be drawn up out of the tank, not down into it.
Understanding the system, will make it a bit easier to trouble shoot.
