Before you go crazy and start taking the inside apart, maybe go on the roof and pop off the cap to the vent and see if you can see if it is blocked from the top. Sometimes insects and critters can create a blockage on at the top of the vent. You could also push a fish tape down the vent from the top to see if you can feel a blockage that way as well.
Also, a telltale sign that a vent is blocked is that the appropriate tank drains slowly or is struggling to drain when dumping. Do you have this problem?
Edit: Wow, take a couple minutes to type a response and get beat to it by several others. Man, I have to learn to type faster.
