Walmart Toilet Chemical vs. RV Dealer Toilet Chemical

Sounds more like a venting problem to me. These systems rely on rooftop vents to vent those tanks. They could be partially blocked or the pipe running from the roof to the tank could be inserted to deep in the tank which when the liquid level reaches the bottom of the vent pipe will force all the gasses produced to push back inside when the toilet is flushed. Is the smell a constant one or happens after some use?