[Richard]

Even after sealing the air admittance valves, we still would get a smell if someone flushed a potty while I was driving. When looked at the Microflush manual, it states in fine print that they recommend a small ventilation fan for sealed tanks. When I look at the way the hopper empties, you may have an open path while flushing. Anyway, I replaced the normal roof vent caps with the windvane type that spins to orient with the wind.

I did that in September, and six weeks and 3000 miles later I am happy to report that it cured the smell issue.

They were cheap, about thirty bucks each. It took more time to get out the ladder than it did to install them.

Here is the type I used. Cyclone vent