Why in the world would you hold it open???
It was meant for it to be a to see visual "example/test" for the RV'ers that do NOT know that the toilet is straight open NO p-trap. :R
Here to tell you. I use chemicals. For years and years no smell. UNTIL I RV'd to Florida in the summer at a site where I had to break camp to dump so I would wait until full.......AND...if you allow the tank to completely fill up, the odor DOES come into the RV when you flush. When you open the flap to dump water just trickles down the side. The time it is open and the time it closes to fill IS enough time for odor to enter the RV.
Again filled tank, 90 degree weather and sitting for days in tank before dumping. Many of us who RV in those conditions will use 2 bottles of chems in that situation.