What Jagiven is calling an AVA valve is actually called an AAV or Automatic Air Vent.
They look like this:
Check under each of your sinks and see if you have something like this. If you do - tap on it, new ones tend to stick open.
You need to do some "sniffing around" and figure out where the smell is coming from. If it's from one of the sinks, check the AVA if it exists, also, if it's a sink you almost never use - pour some water down it to make sure there is water in the trap (that S on it's side shaped pipe under the sink).
One other "dumb question" - do you have pets? If so, have you checked for the obvious? Are they perhaps leaving you with a little present somewhere like behind a couch?
Calgon is a bath soap. The "drop in's" are usually enzymes or chemicals to help break down waste and control odor. Basically people are suggesting soaps and other things to clean out your black tank. But even if the black tank is retaining a poop pile, the smell shouldn't be getting out of the tank.
No offense, but it doesn't sound like you're too much of a "do it yourselfer" so if you can't figure out where the smell is coming from it may be time for a trip to a qualified dealer.