Roof. Pull the vent cap off. See if bees or wasps have built a nest and clogged the pipe. If it looks clear, get an old garden hose and shove it down the vent pipe. If it reaches the bottom it's clear, something else is going on then.
While you have that hose there, you might as well turn the water on and clean your tank out too! If there are any clogs or poop pyramids, this may help break that down.
You could also try shoving the garden hose down the toilet. I'm assuming yours does not drop straight into the holding tank, but goes down a pipe, probably at an angle, and then into the tank ... ker-plop!