I'm also a fan of water only...no rv toilet chemicals. When traveling, the only chemicals I'll use is a cup of Oxy-Clean along with a cap full of Calgon water softener and about five gallons of water. The sloshing of the tank during travel will keep the tank as clean as it's gonna get.
Once at the rv/camp site, I'll drain and flush with about a half tank of fresh water and leave another five gallons in the tank. We use plenty of water during each flush to insure plenty of water is in the tank at all times. Of course, when boon docking, this does create a problem with the tank filling too soon...so you have to make adjustments.
Using chemicals only "masks" the odor and should only be used when trying to find the source of the problem and fixing it. A normal tank does not need chemicals at all.
As others have mentioned check your AAV valve, and replace if in question....they're cheap and easy to install. Checking the top tank vent pipe is easy by putting a water hose in the opening and turn it on. If the pipe is clear, water will come out the tank's drain pipe. I have never heard of any one's vent pipe that "actually" came loose and slipped down to the bottom of the tank... it does sound feasible however.
good luck and let us know what you found the problem to be so that others may benefit from your experience.
Remember, water is your friend when it comes to keeping your black tank happy. Chemicals are for the most part, just wasted $$ down the proverbial toilet. Pun intended, thank you very much.
Ron