Those after-market vent caps work great.
As a seasoned camper, you should already know about things like the water seal in the toilet, and making sure that there is water in the sink trap (if it ties into the Black tank).
I wonder if there isn't a problem with the seal under the toilet? Which might be taken care of, if you used one of those vent caps that actually PULLS the fumes out.
Chemicals are chemicals....I doubt that there is any real difference between the Walmart and other brands. At least not enough to cause these issues. After all, if it was that bad for everyone, who'd buy them?
Good luck:)
