Re sewer smells in the coach while driving on the highway. Most of this has been said already but------
On the road, all the windows and all the roof vents should be shut, and fresh air should be drawn in through the driver and passenger seat cab vents. This keeps a mild positive pressure on the interior of the coach, and prevents black or graywater odours from being sucked in.
It seems natural that the roof vents might draw in more fresh air, BUT at speed they create a significant negative pressure inside the coach which can sometimes draw in waste gases. The coach will draw in from the easiest place, so if you open the toilet valve expect an inrush of 'deathfumes'.
Roof air conditioners, OTOH, and especially the basement models of AC create positive pressure and that is a preventative for ingression, but if it's not warm enough to need AC, just let the cab controls do their work.
When using the toilet, keep the windows and roof vent tightly shut, flush and tidy up the toilet etc, get the valve closed and adequately sealed with water, and THEN open the roof vent for a minute or so to clear whatever miasmas you have made yourself. Then close it again before you go back to your seat.
I'm not against the 360 vents and whatever, but it's not necessary, and toilet chemicals are not necessary either.
Of course you could do what the trains did while I was growing up. Let the toilet drain directly on the track. Ewwww
Rankjo